She'll do anything for love, and she'll kill you if you think otherwise.
A term that usually refers to a character who is a bit too attached to their love interest. Usually, the yandere is female, but there have been male examples. The yandere is in love with someone to the point of obsession. They will do everything from sending love letters and text messages to outright stalking their crush. Sometimes, they will even lock them in a room or physically hurt them if they are afraid that their beloved target is looking at someone else but them. And if some other friend or love interest thinks about making a move on the same person, the yandere will not be afraid to stick a knife in their back to remove the competition or to prove their love to their target. In some cases, the yandere is not afraid of even killing their own crush if they get rejected by him.
It is not entirely clear where the term originated, but the trope still exists in anime/manga today.