Toilet-bound Hanako-kun

If you go to the third stall of the third-floor girls' bathroom in the old school building and knock three times, Hanako-san will appear and grant your wish – for a price. Except when lovelorn high school first-year Nene Yashiro tries, a boy appears from the stall, Hanako-kun. He's willing to grant her wish, but he warns her that the more supernatural the solution, the more bitter the price and that maybe her wish, for the most popular second-year boy in the school to like her, isn't really what she's looking for. Yashiro is stubbornly insistent that it is, however, she agrees to use Hanako-kun's down-to-earth solutions out of a self-help book. When none of those work, though, she seizes on a mermaid scale curse, despite Hanako's warnings, not only cursing herself, but also binding her to Hanako-kun for this life and beyond.

Synopsis from Anime News Network