Imagine Sylar being Peter’s alter ego the way Jessica was Nikki's alter ego:
As a side-effect of his power absorption personality, Peter developed a psychotic split personality with a thirst for the abilities of others which he wants to physically assume instead of waiting to be around others to assume them. But the abilities the Sylar personality takes on only become active when Peter’s Sylar. When he’s not, the powers become inert. The more Peter uses his power absorption personality, the more the Sylar personality grows which causes the wall between it and Peter’s personality to weaken. Peter’s character arc would be him realizing this side of himself, struggling with it by finding ways of suppressing it or trying to stop it from killing innocents, and coming to terms with it by merging with the Sylar personality and using his killing talents in the twisted pursuit of good like Dexter.
As a side-effect of his power absorption personality, Peter developed a psychotic split personality with a thirst for the abilities of others which he wants to physically assume instead of waiting to be around others to assume them. But the abilities the Sylar personality takes on only become active when Peter’s Sylar. When he’s not, the powers become inert. The more Peter uses his power absorption personality, the more the Sylar personality grows which causes the wall between it and Peter’s personality to weaken. Peter’s character arc would be him realizing this side of himself, struggling with it by finding ways of suppressing it or trying to stop it from killing innocents, and coming to terms with it by merging with the Sylar personality and using his killing talents in the twisted pursuit of good like Dexter.