Mine
Mike always hated the idea of ownership in a relationship. Zoey wasn't his property, she was her own person, with her own feelings, opinions, and desires, just like he was. She didn't belong to him and he didn't belong to her. He preferred to think of their relationship as one between two equal partners, working together to make each other happy and make it through life the best way they could.
There was only one circumstance that could make him think the phrase "Zoey is mine." Some of his personalities, especially Vito, were becoming more and more frustrated by the lack of physical contact in their lives. They blamed Mike for not letting them use his body to experiment with others as they pleased. They tried to strike a deal with Mike; Mike's body could stay loyal to Zoey, all he had to do was let the other minds in his head have a turn every once in a while. She wasn't their type, but any port in a storm, right?
Mike drew the line there. He had to share so much in his life with them, but Zoey would never be one of those things, and he knew she'd be just as against the idea as he was. Zoey was his. His disorder took so much away from him, but he'd never let it take her.