A yandere is a character, usually female, who harbors powerful affections for another person to the point of either being jealous, aggressive, controlling, or
downright insane and violent to the point of killing anyone(usually also female) that looks at their supposed
love interest. Typically used to refer to anime/manga characters, but there are similar cases in the real world. (at least according to
2chan)
Basically, a yandere is a word you should
run away from really fast. You may not
live long enough to run, but you will live long enough to try.