"Hanako of the toilet. A ghost that inhabits the toilets of Japanese schools. Usually depicted as a small girl with a bob cut and a red skirt. According to the urban legend, one can summon her by standing in front of a specific stall and knocking on the door several times while calling her name."
Hanako appears as an enemy in St. Hermelin ???. She's an infamous enemy for that dungeon due to her high level and devastating Maeiha spell. This led to her being one of the few demons who had their stats downgraded for the PSP port, as her magic attack was lowered from 71 to 61.
A little girl called Hanako can be found if Seven Sisters High is exited after entering with Eikichi in the neighboring Honmaru Park before defeating Takashi Hanya. There she will explain she passed around Seven Sisters and scared a student who asked her name, mistaking "Hanako" for a small traditional Japanese demon stalking bathrooms of the same name.
Talking to the scared student will prompt Lisa to ask why they don't use the rumors for personal gain; if this is followed through by leaving Seven and spreading the rumor at Kuzunoha Detective Agency, the small demon Hanako will appear in Seven Sisters. Destroying it, collecting its nametag and handing it over to the scared student nets the group a reward of Brink Tea x5.