It must be love.

Romantic and/or sexual relationships between characters are an extremely popular subject in Touhou doujinshi and fanart. Of course, since almost all of the characters are female, "everybody in Gensokyo is gay" is itself a meme. Jokes aside, ZUN himself hasn't officially acknowledged the idea of his characters getting romantically involved as there is no source of him ever saying anything.

This is intended to be a list of the couples most commonly seen in fanon and excludes "crack pairings". Brief explanations of likely canon origins, or at least excuses, are also welcome.

Main Character Harems

As the protagonists, Reimu Hakurei and Marisa Kirisame are frequent subjects of shipping speculation, both with each other and with enemies they've defeated. Marisa, however, is often considered to have a larger "harem". Pairings are roughly organized by order of popularity in each category.

Reimu and Marisa appear as playable main characters, usually both in the same game, in almost every Touhou game, making this the most popular pairing in Touhou ever since the beginning. They are also shown to be acquainted in several places in the games, sharing a sort of on-again-off-again camaraderie. This is a rather popular pairing with fan authors, mostly displaying the contrast between Marisa's sassy, easygoing and spontaneous attitude, and Reimu's more unabashed, laid-back and level-headed demeanor. Artists like Rocket Fuel 21 and Asian Big Brother Generation like to portray an affectionate and aggressive Reimu and a bashful Marisa (or Marisa crushing painfully on her in the second case), but those need to be taken into account as very personal interpretations those authors chose.
The phrase 「僕の見つけた真実はレイマリ」 ("Boku no mitsuketa shinjitsu wa ReiMari" / "My discovered truth is ReiMari") is commonly used to express support for ReiMari.

Reimu Pairings

Reimu and Yukari — the border's shrine maiden and the youkai of the border — made up the Border Team of Imperishable Night. Their relationship in fan works is usually identified by Yukari playfully teasing or flirting with Reimu (as Yukari does poke fun at her a lot in the game), who often acts irritated or impatient with the advances but may become receptive to them eventually.
The phrase 「ゆかれいむは俺のロード」 ("YukaReimu wa ore no roodo" / "YukaReimu is my lord") is commonly used to declare support for this pairing.
Alice appeared to have remembered Reimu from their earlier encounters in PCB and MS (Mystic Square). This pairing was originally somewhat popular before IN's Magic team introduced the Marisa/Alice pairing. However, this pairing's popularity never did die out that much, and remains one of the more popular pairings.
The phrase 「レイアリは原点」 ("ReiAri wa genten" / "ReiAri is the starting point") is commonly used to declare support for this pairing.
Could be said that it simply comes from the fact that the two are both shrine maidens, but the Mountain of Faith dialogues might bear some leads for it. It was the only popular pairing for Sanae before Kogasa x Sanae came about.
The phrase 「レイサナの道を歩む」 ("ReiSana no michi wo ayumu" / "I walk the path of ReiSana") is commonly used to express support for ReiSana.
A pairing that has gained quite a bit of popularity due to their partnership in Subterranean Animism. Other notable encounters are in Phantasmagoria of Flower View in Aya's scenario, where Aya remarks that "A shrine maiden is a great source for information." From regular battles, she also says "Of course, when I'm stuck for stories I'm limited to those of shrine maidens." In Mountain of Faith, Aya also says "It's because I'm the one that knows the most about you." Also, in Reimu's 4th stage in Scarlet Weather Rhapsody, after defeating Aya and the weather changes back to Reimu's weather (which is sunny), Aya remarks that "This is the first I've seen of the sun in a long time. It sure feels nice." Knowing that Aya is always snapping pictures about everything, the people often think that Aya snaps pictures of Reimu while she is alone.
The phrase 「今日のネタもあやれいむ」 ("Kyou no neta mo AyaReimu" / "Today's neta is AyaReimu") is commonly used to express support for this pairing.
In one of the endings of Embodiment of Scarlet Devil, it is mentioned that Remilia often visits the Hakurei Shrine to have tea with Reimu. Naturally, this was taken further.
The phrase 「レミレイが俺のデスティニー」 ("RemiRei ga ore no destiny" / "RemiRei is my destiny") is commonly used to declare support for this pairing.
With no home to go to after the events of Immaterial and Missing Power, Suika was taken in at the Hakurei Shrine. It's plausible to think the oni can get very affectionate at times.
Reimu here is seen as sometimes visiting the Garden of the Sun.
The most popular pairing involving Cirno is actually with Reimu, since Cirno is often seen with Reimu, being bullied as a ⑨. In most versions of the pairing, however, Reimu acts as an older sister.
Reimu is a frequent visitor and freeloader at Kourindou, second only to Marisa.
Koishi's profile in Subterranean Animism says she became attracted to the human who defeated her at the Moriya Shrine. Depending on which character you play as, this could be either Reimu or Marisa.

Marisa Pairings

Alice and Marisa make up the Magic Team of Imperishable Night, and as the two are both witches living in the same forest and have occasional quarrels, they are often shown to be friends. In fan works, the pair is depicted as being somewhat troubled, with Alice hiding her feelings for the often oblivious(?) Marisa and getting exasperated by her attitude. Some works portray a "worst case scenario" in which the anxiety put upon her can become deadly, as she begins to develop obsessive and jealous tendencies that may end up in her harming Marisa or herself. This pairing is popular among fans, bolstered by the popularity of Marisa Stole The Precious Thing. Alice is usually seen as a tsundere that knowing that she is going to die later than Marisa, hides her feelings, but in the end she confess herself. Although there is a possible chance of Marisa becoming a youkai in the future(just like Reimu may become a God as stated by Kochiya Sanae) Marisa is the opposite of Alice in personality, and they fight a lot, so they are seen like a love/hate relationship. In the bad ending of Imperishable Night they share a bed together waiting for the morning.
The phrase, 「マリアリが俺のジャスティス」 ("MariAri ga ore no justice!" / "MariAri is my justice!") is commonly used to declare support for this pairing.
The first time they talked was the Embodiment of Scarlet Devil, stage 5, where Marisa expressed some desire to become a maid (and why would Marisa say she wants to be the head maid?). Additionally, in Immaterial and Missing Power, in stage 3 of Sakuya's route, Sakuya somehow managed to walk to Marisa's shop without getting lost, and Sakuya remarked how she was special for managing to do so. In Phantasmagoria and Flower View, in Marisa's stage 5A, they note that there are no flowers growing in either persons' place of residence. A common depiction is that Marisa still sneaks around the library, and looks for medicinal herbs in the Forest of Magic (with Sakuya helping along).
The phrase 「咲マリは俺の聖域」 ("SakuMari wa ore no seiiki" / "SakuMari is my sanctuary") is commonly used to declare support this pairing.
One of the endings to Embodiment of Scarlet Devil, plus articles from Perfect Memento in Strict Sense and Bohemian Archive in Japanese Red, all make reference to Marisa stealing books from Patchouli's library in the Scarlet Devil Mansion. It is assumed that there is a reason Patchouli tolerates this. In fan works, Patchouli is often shown sharing her books reluctantly with Marisa in order to have a reason to spend more time with her.In the doujinshis and fanfics, she fights with Alice over who gets Marisa.
The phrase 「生涯パチュマリ一筋」 ("Shougai PachuMari hitosuji" / "Lifetime PachuMari resolution") is commonly used to declare support for this pairing.
Upon defeating Flandre in Embodiment of Scarlet Devil, Marisa suggests for Flandre to get married, with Reimu for example. At times, fans will couple the young vampire with Marisa instead (actual Flandre x Reimu pairing is very uncommon, almost never seen), though the latter is still the one who gave her the hint.
The phrase 「フラマリが私の人生」 ("FuraMari ga watashi no jinsei" / "FuraMari is my vitality") is commonly used to declare support for this pairing.
Rinnosuke has known Marisa since she was a child, worked for her father, and created her mini-hakkero. The two are occasionally portrayed as harboring affections towards each other.
This pairing was inspired by the option of choosing Nitori as Marisa's youkai partner in Subterranean Animism's Marisa type C firing mode.
The phrase 「今日の実験はにとまり」 ("Kyou no jiken wa NitoMari" / "Today's experiment is NitoMari") is commonly used to express support for this pairing.
This is actually more popular than Yuka x Reimu. Probably due to the notion that Marisa's trademark Master Spark was originally used by Yuuka, who would be much less than pleased about the issue.

Masters and Servants

The suffering and frustration experienced by serious-minded servants at the hands of their exasperating, capricious masters is a frequent source of humor in both official works (especially Inaba of the Moon and Inaba of the Earth) and fan works. The following are arranged roughly by order of popularity.

Many theories have been proposed to explain why Sakuya is so devoted in her service to Remilia. Is she a defeated vampire hunter? Does Remilia feed on her blood? Is she just a lolicon?
The phrase 「レミ咲が今日のティータイム」 ("RemiSaku ga kyou no tea time" / "RemiSaku is today's tea time") is commonly used to declare support for this pairing.
This pairing can be depicted in several ways, but most often seen with Meiling caring for Flandre in a motherly fashion, though fanon tends to take this further. Additionally, the two are seen as colorful characters, due to the nature of Meiling's rainbow-ish spell cards and Flandre's rainbow wings.
The phrase 「メイフラは希望の虹」 ("MeiFura wa kibou no niji" / "MeiFura is the rainbow of hope") is commonly used to declare support for MeiFura.
Nazrin is Shou's underling and also the midboss in her stage. Similar to Ran and Chen's pairing, Shou is depicted as motherly or rather just romantic toward Nazrin. Though unlike Ran and Chen, Nazrin has shown tsundere tendencies towards her employer.
Eirin's devotion to Kaguya is such that she is willing to do anything Kaguya asks of her, including violating the laws of the Moon to make her immortal.
The phrase 「えーてるは心の癒し」 ("E-Teru wa kokoro no iyashi" / "E-Teru is the solace of the heart") is commonly used to declare support for E-Teru.
Youmu projects the image of a loyal samurai, but she's also a young girl who frequently gets flustered at Yuyuko's carefree personality. As seen in Silent Sinner in Blue, Yuyuko also has the best awareness of the plot and provides Youmu with expository explanations.
The phrase 「ゆゆみょんは俺のファンタジー」 ("YuyuMyon wa ore no fantasy" / "YuyuMyon is my fantasy") is commonly used to declare support for YuyuMyon.
The fanon interpretation of Momiji's personality is very similar to Youmu's: serious about her duty as a guard and skilled with the sword, but innocent and naive in most other respects. This makes her a frequent target for Aya's teasing and abuse.
The phrase 「文椛は自然の摂理」 ("AyaMomi wa shizen no setsuri" / "AyaMomi is nature's providence") is commonly used to declare support for this pairing.
This was easy to predict, since Shou was the original helper of the sealed magician Byakuren. She's depicted with Shou in a parent-like role (as she's shown toward every other boss in UFO) or sometimes a romantic one, since their stages are right next to each other (Ultimate and Penultimate boss pairing).
Ran is usually the only one keeping the Yakumo household running because Yukari is asleep most of the time.
The phrase 「ゆからんは境界を超える絆」 ("Yukaran wa kyoukai wo koeru kizuna" / "Yukaran is a bond transcending boundaries.") is commonly used to declare support for this pairing.
Kanako is dominant over Suwako, having taken control of Suwako's shrine by force, and Sanae is often depicted as being a level-headed shrine maiden who tries to keep both of her goddesses' antics under control.
Mima is considered to have been a mentor or motherly figure to Marisa during the PC-98 era.
Satori is shown to be very devoted to her pets, even when they get out of control sometimes.
Koakuma is believed to be a demon—possibly a succubus—that Patchouli summoned and bound to serve her as assistant librarian.
Byakuren is considered the "granny" of the UFO team, and Nue and Murasa being the children of the team, so they often fight over who likes Byakuren better. Frequently, things get messy between Nue and Minamitsu, sometimes turning into something more...seductive and flirtatious.
In addition to the popularity of RemiSaku and MeiSaku, it is also known that Meiling has been serving Remilia far longer than Sakuya.
In this case, it's Shikieiki who's serious and Komachi who's the flaky servant.
The phrase 「こまえーきは我が悲願」 ("KomaEiki wa ware ga Higan" / "KomaEiki is my Higan"; note: Higan means "one's dearest wish") is commonly used to declare support for this pairing.
Supposedly, Alice is from Makai, created by Shinki.
Ran is often depicted alternatively displaying a motherly or a lolicon attitude towards Chen.
It's a PC-98 relationship between two humans, Yumemi being the older one. She often shows Chiyuri (her assistant) a rather abusive side, which fanon has taken further, to the point of Yumemi being tsundere for Chiyuri. In other words, a usual Master-Servant relationship, but in a more human way.

Sibling Pairings

There are often pairings in which two sisters of the same family name or from a different surname are depicted.

The two Scarlet sisters and most known vampires in the series.
The most popular pairing that feature either of these characters. In fact, it is pretty much the only mainstream pairing involving either of these characters.
Flandre and Koishi are both EX-bosses and both younger sisters of their loving older sisters (Remilia and Satori). A few fans suggest that Koishi is the one visiting Flandre to play with, or having a relationship since their older sisters are "busy" with their servants.
The phrase 「こいフラは俺のイドブレイク」 ("KoiFura wa ore no id break" / "KoiFura is my id break") is commonly used to express support for this pairing.
This is the only pairing that exists for either of the Aki sisters.
Both are or were married. However, their husbands are never seen, bear little mention, and have no major role in the story, only serving to establish the Watatsuki's relation to Eirin. Meanwhile, the sisters are often seen together and share a close bond, sometimes taken to mean more by some.

General Pairings

Some other pairings that may be canonical or don't fit anywhere. Pairings are arranged roughly in accordance to popularity, top first.

Keine appears in Imperishable Night's Extra stage to protect Mokou, when she said 「あの人間には指一本触れさせない!!」 or "you won't lay a single finger on that human!" Of course, 「あの人間」 or "that human" could refer to nobody other than Mokou.
The phrase 「もこけねは神の国」 ("MokoKene wa kami no kuni" / "MokoKene is the land of gods") is commonly used to declare support for MokoKene.
Yuka is incredibly powerful, so it wouldn't make sense for her to pay attention to lower-level youkai. But Wriggle is a bug. Bugs pollinate flowers, and Yuka is a flower youkai. ANGELTYPE's depiction of this pairing (in Nightbug and Flowerland) features Yuka acting as protective big sister for Wriggle, or alternatively showing her affection by tormenting her.
The phrase 「ゆうリグは俺の向日葵」 ("YuuRigu wa ore no himawari" / "YuuRigu is my sunflower") is commonly used to declare support for this pairing.
Sometimes, the two pets are seen to be affectionate with each other. Canonically, Orin triggered the events of Subterranean Animism because of concern over her friend Okuu.
Yukari is a longtime friend of Yuyuko, having known her even before her death.
The phrase 「ゆかゆゆが俺の幻想郷」 ("YukaYuyu ga ore no Gensoukyou" / "YukaYuyu is my Gensokyo") is commonly used to declare support for this pairing.
Their stages are next to each other. It is only a minor connection, but enough to make it popular enough that it is almost the only pairing that features Hina.
The phrase 「にと雛が俺のウオオー!」 ("NitoHina wa ore no uoo-!" / "NitoHina is my uoo-!") is commonly used to declare support for this pairing.
Both are handy at a particular ability, as maid and doll-maker, as well as having other kinds of similarities.
The phrase 「咲アリが俺への殺人ドール」 ("SakuAri wa ore e no satsujin dooru" / "SakuAri is the killing doll") is commonly used to declare support for this pairing.
A pairing that has become popular for some reason, most likely due to fanon's interpretation of Alice as a more submissive individual, as opposed to Yuka's dominant nature.
The phrase 「幽アリは七色の花畑」 ("YuuAri wa nanairo no hanabatake" / "YuuAri is a rainbow-coloured flower garden") is commonly used to declare support for this pairing.
Probably the bonding together of oni companions.
They are both sorceresses who are shut-ins. Additionally, they're both seen as Marisa-obsessed girls in Fanon, which has lead people to think that Marisa can't handle her "harem".
The phrase 「パチュアリは真理」 ("PachuAri wa shinri" / "PachuAri is the truth!") is commonly used to declare support for PachuAri.
In Double Spoiler Hatate appears as Aya's rival. The pairing is clearly a form of rivalshipping.
Despite no certain connection between the two, the fact that they appear on the same stage has led some to couple the two together.
The phrase 「大チルが俺のアイシクルフォール」 ("DaiChiru ga ore no Icicle Fall" / "DaiChiru is my Icicle Fall") is commonly used to declare support for this pairing.
Canonically, Tewi often plays practical jokes on Reisen or frames her so that Eirin can punish her. (See below under "Domination or Rape")
They are both fifth-stage bosses, and in Imperishable Night, Youmu concludes that Eirin is similar to Yuyuko in regards to treatment of their servants. There is also a contrasting nature of their weapons, where Youmu uses a sword, whereas Reisen uses something very bullet-like.
Often features the two acting as "parents" for Sanae.
They are co-workers for the same ship, and additionally, in Undefined Fantastic Object, Murasa was the captain and Ichirin was the guard of the ship, and they have also know each other for more than a thousand years. Reimu A's ending mentioned that both were sealed in the depths of the earth. Additionally, they both appeared in level 5 in Double Spoiler.
Patchouli is a close enough friend to Remilia that Remilia has allowed her to live in her house, probably for centuries. In fact, as seen in Bohemian Archive in Japanese Red, they call each other by their abbreviated names. This suggests some level of intimacy.
As seen in the Scarlet Weather Rhapsody, Iku acts as a guardian for Tenshi whenever she does mischief, causing Iku to punish her. And since Tenshi is a masochist in fanbase.... However, there are rare cases in which Tenshi is dominant over Iku.
In Curiosities of Lotus Asia, Yukari is seen to visit Rinnosuke's shop, and they have a common interest in artifacts from the outside world.
In canon Nue mentions Murasa by name. Murasa's name also resembles that of the archer who supposedly sealed the mythological nue. The pairing seems to have spun from there.
The only two individuals known from the world past Gensokyo's border, in a night-activities club that consists of only them.
The two together are commonly called 「秘封倶楽部」 ("Hifuuku Gakubu" / "Secret Sealing Club Music Department.")
The fact that they are residents of the same mansion has led a few to pair these two together.
Rinnosuke is half-human, half-youkai, the major theme here. It's commonly thought that he would have more in common with Youmu or Keine because of this. Sakuya and Youmu are also semi-frequent visitors to Kourindou. Rinnosuke, Keine, and Akyuu are all connected with the Human Village. When paired with Keine, the common setting is both being childhood or long time friend. The Hieda, in particular, seem to be well acquainted with him. In Sanae's case, Rinnosuke is heavily interested in items and matters of the outside world, and because Sanae is from the outside, she would be the most interested or know the most about the things he carries.
In fanon, Cirno looks up to Letty as a motherly or big-sister figure whom Cirno can only see in winter, while Cirno exists in Gensokyo year-round.
The phrase 「レティチルは雪の結晶」("RetiChiru wa yuki no kesshou" / "RetiChiru is snow crystals") is commonly used to declare support for this pairing.
Similar to Meiling x Flandre, it is commonly said that the place where Meiling holds guard is a place where "children are helped and taken care of by a gentle onee-san."
The phrase 「めーちるが俺の冷やし中華」("MeiChiru ga ore no hiyashi chuuka" / "MeiChiru is my chilled China") is commonly used to declare support for this pairing.
A pairing that has gained some popularity due to some hints in Suika's ending in SWR.
They are both residents of Koumakan, and this is sometimes seen alongside RemiSaku, although there is probably no other reason for this pairing.
Other than the fact that they are both residents of Koumakan, this pair was made popular by Patchouli&Meiling M-1 Grand Prix which was titled "PachuMirin" - hence, the strange name for this pair.
There is nothing connecting the two, but both are seen as childish sometimes, so they are also commonly depicted as being playmates.
Both live in the forest, with Alice often depicted visiting Mystia's stand or helping there.
A pairing popularized by Mitsumoto, despite no canon connecting the two. Usually has Mystia admiring Mokou, who may or may not answer her feelings. Supporters of this relationship will change Mokou's occupation to something other than a yakitori vendor, such as selling charcoal for Mystia's lamprey stand.
The phrase 「もこみすは盲目の八目鰻」 ("Mokomisu wa moumoku no yatsumeunagi" / "Mokomisu is the lamprey of blindness") is commonly used to express support for this pairing.
Despite no indication that these two people are separate people, it has been common to couple them as if they were separate. The theme of "black and white" is common with this pairing.
Another example of the trend in shipping consecutive low-level bosses.
This is due mainly to their conversation in Immaterial and Missing Power as well as Scarlet Weather Rhapsody.
A relationship between the "intelligent" characters, based from one 4koma.
Both were nameless midbosses in Embodiment of Scarlet Devil.
Both are known for being powerful youkai.
The origin of tengus is the god Sarutahiko and the origin of mikos is the goddess Ama-no-Uzume - according to mythology, these two were a married couple. As a result of this lineage, we have this pairing between the tengu Aya and the miko Sanae. Both have wind-based powers, and are bosses of stages 4 and 5 respectively in Mountain of Faith. This pairing was popularized by pasteltel.
Mokou's name (妹紅) could mean "Scarlet (younger) Sister" or "Paint me red". Flandre is the younger sister of Remilia Scarlet, and both are EX-bosses. This pairing was quite popular before pairings with Koishi and Nue arrived.
Comes from the events following Cirno's Phantasmagoria of Flower View scenario.
The phrase 「アヤチルは俺の⑨面記事」 ("AyaChiru wa ore no kumen iji" / "AyaChiru is my ⑨th page article") is commonly used to declare support for AyaChiru.
From her small appearance in CiLR, she's since come to hang around Kourindou. Often argues with Rinnosuke for the return of her stolen books, which are currently in his possession.
The two as sometimes seen as opposites, although there is nothing canon to connect the two.
Though it is unlikely that the old Alice (i. e. "Lolice") and the windows version of Alice are two different people, sometimes they are portrayed together as separate people.
Due to the fact that Shangha is Alice's doll. Sometimes, Alice's dolls are depicted as autonomous living beings.
As the goddess of misfortune and the misfortunate bridge princess, they are bound to have something in common. (More information is needed here).
The phrase 「雛パルは不幸中の幸い」 ("HinaParu wa fukouchuu no saiwai" / "HinaParu is the silver lining of a dark cloud") is commonly used to declare support this pairing.
Mainly due to their contrasting natures of fire and ice.
Momiji is known to play Dai-Shogi against kappa in her free time, but Nitori is the only kappa character we know of so far.
There is probably no reason for this pairing other than the fact that they are both residents of Koumakan, and of course, aesthetic reasons.
There is nothing that connects the two together, aside from the fact that the two are commonly seen as "delicate." It is sometimes found along with ReiMari.
Perhaps started because of Rinnosuke's love of reading and the printed word. Aya is the main supplier of such in Gensokyo, so Rinnosuke is commonly depicted as being one of her (few) subscribers.
The doll-maker and the living doll. They were seen talking during Remilia's party in Silent Sinner in Blue.
Kogasa is the mid-boss of Nue's stage in Undefined Fantastic Object, even though she has no relation to her or the plot. Because of the trend of pairing closely grouped bosses together, fanarts like this became the result shortly after the game's release. Additionally, they are both youkai that enjoy scaring, or at least surprising, people.
Yuugi has many things for Parsee to become jealous over: strength, height, fighting skill, breast size.... As with Hina and Nitori, there is nothing actually to connect the two, yet the pairing turned popular enough to get lots of fan art and few doujins. Usually features Parsee as a typical tsundere and Yuugi as the more aggressive one.
The phrase 「勇パルは俺のジェラシー」 ("YuuParu wa ore no jealousy" / "YuuParu is my jealousy") is commonly used to declare support for this pairing.
The most popular pairing featuring Medicine, for some reason. Due to they have an ending in PoFV which involves the two together
Spread of disease and spread of poison. In addition, they are also both depicted (in fanon) as outcasts, leading to a pairing.
The phrase 「ヤマメディが俺の闘病日記」 ("YamaMedi ga ore no toubyou nikki" / "YamaMedi is my diary on my fight against an illness") is commonly used to declare support for YamaMedi.
The similarity of Nitori's hairstyle to that of PC-98 character Chiyuri has led to jokes about them forming a "Twin-Tail Alliance". Their shared interest in science has only furthered their pairing status.
Both darkness-related, also, Rumia has a spellcard named "Night Bird".
Due to the fact that their powers are polar opposites (fire/ice) and the fact that they are both portrayed as stupid. It also serves as the basis for the "cold fusion" pun, and ⑥⑨ (⑥ representing Utsuho and ⑨ Cirno).
Fighting side by side in the Extra Stage of Legacy of Lunatic Kingdom.
Both have their power revolved around reversing situation.

Domination or Rape

As often seen, but may also be depicted in romantic and consensual contexts. These are roughly arranged by order of popularity.

Usually involves Meiling having an unrequited crush on Sakuya and Sakuya frequently stabbing Meiling in the head for not doing her job as door guard. However, the violent factor of this relationship has become less prevalent lately, especially with some fanart depicting a younger Sakuya looking up to Meiling like an older sister.
The phrase 「世界美咲劇場」 ("Sekai MeiSaku Gekijou" / "World MeiSaku Theatre"; note: 「名作」 or "meisaku" means "masterpiece") is commonly used to declare support for MeiSaku.
Canonically, both of them continually fight each other with deadly force, even though they're immortal and regenerate all damage. In Inaba of the Moon and Inaba of the Earth, Mokou seems to behave like a tsundere as well, and Kaguya genuinely likes Mokou. A possible result of this is that more works that explore this relationship nonviolently have been appearing, many portraying Mokou as a tsundere confused by her feelings, but more recently the idea that Mokou might redeem her family honor by wooing the princess who violated it has been gaining ground.
The phrase, 「てるもこは永遠の輝き」 ("TeruMoko wa eien no kagayaki" / "TeruMoko is the radiance of eternity" is one commonly used to declare support for TeruMoko.
A pairing that has become somewhat popular due to the fact that Sanae said harsh things to Kogasa in the dialogue of UFO. Right now, it has become the second of the 2 main pairings for Sanae, and it may eventually overtake Reimu x Sanae in terms of popularity.
The phrase 「こがさなは俺の相合傘」 ("KogaSana wa ore no ai-ai gasa" / "KogaSana is my umbrella-sharing") is commonly used to declare support for this pairing.
Cirno freezes frogs, and Suwako has relations to frogs. This often causes them to fight, with Suwako winning most of the time.
The phrase 「すわちるは自然界の掟」 ("Suwa chiru wa shizen-kai no okite" / "SuwaChiru is the law of nature") is commonly used to declare support for this pairing.
Similar to the previous pairing, but usually depicted with more extreme circumstances. It was due to how both of them were loyal servants of their masters (as well as the fact that both of them play homage to Jojo's Bizarre Adventure and are said by very few to be like sisters). Popularized by Sougetsu-Tei, after which "Fangirl Youmu" became a meme unto itself, with pictures of Youmu being photoshopped with a lovestruck expression wherever possible.
Eirin is frequently shown, even in the canonical Inaba of the Moon and Inaba of the Earth, to subject Reisen to medical experiments and corporal punishment. As a side note regarding Kaguya, Reisen's occupation as seen in the profile is "Kaguya's pet".
Yukari is one of the strongest youkai. Tenshi is depicted as a masochist who keeps asking for punishment. You might see where this is going...
Similar concept to Yukari x Tenshi: Yuka is known for being the strongest youkai and for having painful battles.
An extension of the meme about Yuyuko trying to eat Mystia.
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