Yandere

A yandere is a type of character in anime and manga characterised by being affectionate, becoming either possessive or obsessive. The term is a portmanteau of the Japanese yanderu, meaning mentally or emotionally ill, and deredere, lovey-dovey, to show genuinely strong romantic affection. Yandere characters are often willing to go to extreme lengths to be with their lover, and have been said to be reminiscent of people with borderline personality disorder; the article's example is Yuno Gasai, a character violently protective of her love interest. The article is a short stub and names no prose or serial examples.

For writers the yandere is the dark end of the affection dial where the tsundere sits nearer the middle: devotion as menace rather than charm. Handled as horror it is a potent stalker engine; handled as romance it risks romanticizing abuse, so the frame the story puts around the obsession is the whole decision.