noun
multiple personality; split personality
Refers to the psychological condition now more formally called Dissociative Identity Disorder (DID). In everyday Japanese, it can also be used loosely to describe someone who seems to have very different sides to their character.
彼は多重人格と診断された。
He was diagnosed with multiple personality disorder.
あの人はまるで多重人格みたい
That person changes their attitude as if they have a split personality.