I'm a relatively self-contained being distinct from the one known as Kaethonry. I ain't him and he ain't me. We aren't entirely separate—We can recall eachother's memories, though it feels different than recalling our own. But I have my own sense of identity, my own accent, my own temperament, and I can access my skills easier than Kaethonry can access mine (If he tries to do my Esperanto Duolingo lesson, he does a mix of German and incorrect Esperanto). Even when I'm out of touch with my identity, the other stuff remains in place, evidently meaning that my existence goes beyond the surface level—I have my own subconscious that knows who it's supposed to be even if I don't!
As far as separation in sense of self goes, we are different people, and I like it that way! It feels horrible when Kaethonry's selfness leaks into mine. I like being me and not being him! Not that it's bad to be him, but I'm grateful to be given space to exist as my own person (and in exchange, Kaethonry gets the benefit of higher happiness). But as far as development goes, maybe I'm not a full person, at least not yet. It's somewhat comforting to me, to be able to fall back on the idea that we collectively make up a whole person, and I don't have to be a person all on my own.
Yet, I don't feel at ease being called a "subpersonality", though I'dn't feel offended by it as long is it wasn't meant as an offense. Why? Well, it implies that that "whole person" is anything more than a metaphor. There isn't a literal overarching person who identifies as both me and Kaethonry while having their own personhood. There's just me and Kaethonry (and some others) existing side by side, who are ourselves and not eachother.
I like "personality" as a term to describe me, because it respects my individuality while giving me permission to not have to live up to the pressure of being a person, when being a person feels like too much of an ask in the moment. I can just exist without me or Kaethonry (mostly Kaethonry) having to worry if I'm "really" a person.