Depends on the personality. They are a person after all, and everyone is different. He could become hurt and angry, he could feel lonely and depressed, he could not care either way and fall in love with a different person causing possible cheating, worse case scenario (or best depending on how the personalities get along) one of them disappears. In this story I feel like he would keep fighting for love as well, eventually becoming depressed. If finally loved back YAY HAPPY ENDING, if not then he'll probably just disappear, maybe even forever.
I happen to be a split personality. We either find someone who we both love and loves both of us, or we leave the person.
To be honest I think the uke should just accept every personality of his lover. He should love them all the same 'cause they're all parts of him.
It's ridiculous to say he prefers one of his personalities when they're all the same person...