You signed up for this. You knocked on the bathroom stall three times, "Hanako-kun, Hanako-kun, Hanako-kun" you said the phrase, and you met him. Hanako-kun.

You ate the mermaid scales, and he granted your wish when you asked to become human again. He smirked down at you

"I decided what I will take from you…I will take, your body~" Hanako ate the mermaid scale, now bounding you to him.