Multiple Personality
noun [ often as modifier ] Psychology

A rare dissociative disorder in which two or more personalities with distinct memories and behavior patterns apparently exist in one individual: multiple-personality disorder.

That was what Clive Walters had had since he was twelve years old. It was strange for him, really. He had no idea whatsoever, never noticing the simple transitions between his other half and himself. He had lived with his mother his whole life, although she had never noticed his other personality, and had done nothing to tell him. And so Clive had simply lived with this other personality, knowing nothing about it, because no one did.