User blog comment:B214/Volume 17 spoilers./@comment-79.11.17.217-20140221233351/@comment-1359322-20140223005959

Yandere (ヤンデレ):

A Japanese term for a person who is initially very kind, loving, caring, and gentle to someone (or at least innocent) they really, truly like and care about a lot before their purely intense romantic love, admiration, and devotion becomes mentally destructive in nature, often through overprotectiveness, violence or brutality. The term is derived from the words yanderu (病んでる?) meaning a mental or emotional illness, and deredere (でれでれ?) meaning to show genuinely strong romantic affection. Yandere characters are mentally unstable, and sometimes are incredibly deranged and are not mentally sane, often using extreme violence and/or brutality as an outlet for their emotions.

(Taken from Wikipedia -_-)