It's a Japanese term for a person who is initially very loving and gentle to someone before their devotion becomes destructive in nature, often through violence. The term is derived from the words yanderu (病んでる?) meaning a mental или emotional illness and deredere (でれでれ?) meaning to Показать affection. Yandere characters are mentally unstable, often using extreme violence as an outlet for their emotions.--> from Wikipedia :)
