Shino

It is implied that Shino was a patient in a psychiatric hospital, the girl injuring herself. She has been manipulated by Mukade, her doctor, who made her enter a cult with some strange rituals. Her family was also his victim. After being ruined, Shino killed herself.

Mukade probably told her to commit suicide, manipulating her. As some other Wonder Egg Girls, she also tried to persuade her protector, Rika Kawai, to abandon the fight and die. Since she also suffered from suicidal intents and self-harming, It was easier for Shino to try to eliminate Rika, as she presented some similarities with this character. Although she was still manipulated, Shino was clearly on her killer's side.

(Source: Wonder Egg Priority Wiki)