Oni-Gokko(2006)

Oni-Gokko

All Yoriko could remember from the night before was the red blood, the red moon and an exceedingly bewitching red eye... The next day during class, being on the verge of remembering leaves her in a sickened state and her classmate Kirin suggests she take a nap in the nurse's office. When Yoriko arrives she notices nobody is around so she decides to make use of the empty bed.

While resting, the absent nurse returns to the office to finish the job she had failed the night before: suck Yoriko clean of blood. In the middle of the struggle, Kirin comes to save poor Yoriko from the demon nurse.

Turns out Kirin and her brother Koryu are 'oni'.