Japanese term for a personality that is initially extremely loving and gentle, then at one point becomes deranged or psychotic, often brutally so. You can cosider it the opsite of tsundere.
Anime and manga intended for the adult male demographic.
Yandere refers to a person who starts out sweet but ends up being obsessive and often violent with their emotions towards people. Their complete devotion to another character becomes deranged, devastating, and destructive.