She had an ability that allowed her to see entities that resembled spirits and ghosts. She was seen as insane and stupid by her parents and teachers, leading her to be sent a psychiatric hospital. Yae was the only one to have that power, feeling afraid of what she saw and being lonely, unable to confide her sufferance to anyone as nobody would trust her.

Due to the stress she experienced, she decided to commit suicide as a way to free herself from the shackles that have weighed her down for too long.

Ai was the first person to believe her, experiencing the same power for a short time. She was able to see Kaibutsu Shoujo, Yae's Wonder Killer, after taking her beaded bracelet.

(Source: Wonder Egg Priority Wiki)