I thought this was gonna be a "dark romance" (idk why did I think that) but this is just pure dark, no romance at all. However, the Uke has a kind of point; they're not dating, and their relationship isn't clear enough for the Uke to understand because he's stupid.
Most of the time, the second or other personality was created to "protect" themselves or something. So, I'm curious why this dude has this kind of personality.
Trust that woman, don't trust that Sera girl.