Gem

Yuta Okkotsu is a nervous high school student who is suffering from a serious problem—his childhood friend Rika has turned into a curse and won't leave him alone. Since Rika is no ordinary curse, his plight is noticed by Satoru Gojo, a teacher at Jujutsu High, a school where fledgling exorcists learn how to combat curses. Gojo convinces Yuta to enroll, but can he learn enough in time to confront the curse that haunts him?

Rikka is a ninja descendant. Her grandfather Hyoe is the last chief of their ninja clan and her father is a cyborg researcher. Both are killed when a weapons' company raids his lab to use his research. Rikka sets out to avenge their deaths.