Yandere (ヤンデレ)

Noun
(plural yandere)

(chiefly Japanese fiction) A fictional character who fits the archetype of being genuinely romantic, loving, kind, merciful, sparing, sweet and gentle, but is at the same time brutal, psychotic or deranged in behavior. The psychotic tendency can be both sudden and ever-present. Often used for both comedic and dramatic displays of character.
plural form of yandere
Origin
Yandere (ヤンデレ ) is a portmanteau of two Japanese words yanderu (病んでる ), meaning to be sick, and deredere (デレデレ ), which is defined as strongly and deeply exhausted, infatuated, moonstruck, head-over-heels, or lovestruck, but in this case used for "lovestruck."

The word was developed from tsundere which describes a character who is both hard-nosed or moody tsuntsun (ツンツン ) and the aforementioned lovestruck. The sick portion was added when a new layer of romantic obsession came about beyond normal tsundere (where cool emotions were warmed and nurtured towards awkward, romantic or sexualized tension) where seemingly normal displays of strong and deep romantic love and affection become mentally dilapidated (i.e. kidnapping crushes, poisoning food that is to be eaten by a romantic rival, or forcing the romantic love interest to commit shinju (心中 ))