dnd 5e – How does leveling work for a character with multiple personalities?

My brother wants to play a character with several personalities, one of the personalities being a craftsman and the other a rougue, but he does not know how to level the two personalities, as he only gives the personality xp control at that time that the character is undervalued compared to the party, which is not good, especially since it is a character for a campaign to the grave of annihilation.