“Yandere” is derived from the Japanese words yamu, meaning insane or sick, and deredere, meaning affectionate or loving. Simply put, a yandere is someone who is lovesick, someone who has been driven to insanity by extreme obsession or love, thus resulting in abnormal behavior if not violence.

The word “yandere”, a term that blossomed in moe fandom, refers to a character who is crazy about someone else… often literally and violently.

– excerpt from tvtropes yandere article