Amy
Amy is Alice Margatroid's half sister who was, for a long time, in a rivalry with Marisa Kirisame. She only appears in the fangame Embodiment of Scarlet Devil Deluxe with Alice. Amy is also going to be in the IOSYS based movie Marisa Stole the Precious Thing, in theaters Spring 2020.
History
In EoSD Deluxe, Amy is introduced when she and her sister are accidentally put under Marisa's "Wave Spell." When the player defeats them, Amy will introduce herself and inform of the rivalry between her and Marisa. She later appears in the Extra stage under the "Flame Spell," cast by Flandre Scarlet to make her a helper; but Alice and Marisa free her.
In the IOSYS movie, Amy watches Alice under the "Love Frenzy" spell, though Amy is not under it. This reminds her of her rivalry with Marisa and she finally breaks the age-old feud to convince Marisa to return Alice's feelings, and also confess her precious feelings for Alice.
Friends and Rivals
Letty Whiterock was Amy's friend as stated in EoSD Deluxe, but it's unknown what happened to them.
She's known to be friends with Alice. In fact, they're sisters!
She used to be friends with Marisa, but again, they broke up and became rivals until the movie Marisa Stole the Precious Thing.
Cirno, Sakuya Izayoi, and Remilia Scarlet are some of her other friends as seen in EoSD Deluxe and Marisa Stole the Precious Thing.
She's not friends with Flandre, after the events of EoSD Deluxe Extra.