Which one, you ask? I'm not sure myself.
EDIT: Several users have informed me that this kid is dressed up like Winry Rockbell from Fullmetal Alchemist. I see what they mean!
If Valentine's Day is about expressing feelings, and the most powerful feeling is sadness/despair/misery, why wouldn't you send your Valentine something to remind them of a horrible, powerful moment from one of their favorite anime?
WARNING: SPOILERS AHEAD