You said they were opposites. That they destroy each other if they come into contact. If he was with you from the start, then you wouldn't have been born. That means you belong together. He can't make you disappear any more than you can make him go away.

[ On the other hand... ]

If he came after, then the moment you came into contact, you would have destroyed each other. But you're still here. He can take over you and not be destroyed by your Innocence. That's likely why you can house him and not be destroyed by what makes him a Noah.

[ His fingers start to warm the more he thinks about it. He's not good at speaking off the cuff, his thoughts are always a jumble he has to sort out, but it all falls into place in front of him, given his background. ]