Obsessive Love (or Obsessive Love Disorder) is a condition where a character has been driven into insanity by an overwhelming obsessive desire to possess and protect another person, with an inability to accept failure or rejection. Depending on the intensity of their affection, obsessive lovers may feel entirely unable to restrain themselves from extreme behaviors such as acts of violence toward themselves or others.
In the anime and manga fandom, including danbooru, the term yandere (from yanderu, meaning mentally sick, and deredere, meaning lovestruck) is used to describe that character trait (and sometimes an over-dramatization of it). Usually, they are extremely brutal and do not hesitate to hurt and/or kill others in order to be with their beloved.
This tag is for characters who go insane from obsessive love. If the character in question is insane because of other reasons, use crazy instead.