Bim is so similar to how Damien once was - kind, trusting, undyingly supportive of his friends. Does Dark see that similarity?
What if all of the Egos are Marks from different universes just like how our Mark is different from the Mark that stole Damien’s body, so Wilford and Dark have been universe-hopping looking for the one that betrayed them, gathering an army along the way.
And Dark found Bim one day.
He wasn’t sure that the bubbly television host could be of any use to him at first, but he brought him along for that very reason, because Bim reminds Dark of who he once was. And then he introduced Bim to Wilford to be his right-hand man.
Dark found a replacement Damien to keep Wilford happy.