Master of Martial Hearts

Aya Iseshima is walking home from school when she comes across two girls fighting viciously for something dubbed the "Platonic Heart." Aya, who is a martial arts expert herself, steps in to break up the fight. Aya later befriends a fighter and learns that a battle royale tournament is taking place for ownership of the Platonic Heart, a diamond that can grant one wish to every girl who possesses it. Unfortunately for Aya, it appears that her meddling has made her a participant in the conflict and that she has no choice but to prevail.