[quote=@Dragonfly 9]
[@CaptainManbeard] Or I could have it as a completely original "personality" that just seems to want to torment the teen at first, and then has an identity crisis as its personality starts to develop beyond its original "purpose".

Decisions, decisions. xD
[/quote]

Maybe an inner sense of guilt manifested? You could add some past trauma, like a loved one dying in a car crash, and he has internally blamed himself for years, which gave birth to the hallucination of a tormentor who then gains sentience? Like, when he was in school trying to focus on a test, there could be the silhouette of a man standing in the corner who whispers hateful taunts at him, telling him it's his fault their loved one is gone. Then when he begins to gain sentience, he could have severe anger issues at first until he works through them. Prior to him getting his own permanent body, he could take on the external form of a floating misty wisp-like thing that follows the group around, but he has to stay close to your character as long as the gland is still in his head. Or the entity could be something like a wizard's familiar to him until they are fully separated. If you go with the dragon egg idea, it could hatch and the other personality is in the body of a baby dragon. Once the dragon begins to grow, your character could eventually ride it as a flying mount or something, lol.

Just some ideas, ultimately it's up to you XD. But I'm excited regardless!