Every time you look in the mirror you see yourself. You don’t think much of it, you hair might be messier than usual, but it’s the same as always. You are you. Now, when someone else is out and about, they think they look differently than what they actually look like.
Instead of having long brown hair, they think they should have shorter curly red hair.
Instead of being short, they should be 6″2.
Instead of being human, they’re a demon.
Physical descriptions change drastically among personalities, and this can cause issues. They might want plastic surgery to fit their ideal looks, they might get tattoos, or spend money on clothing that you don’t really want. I suggest, instead of trying to control what they do, let them change their hair, let them express themselves. I’m not saying get plastic surgery, but if you’re open to the idea of tattoos, find one you both can agree on. Personalities have a lot of issues with this, with wanting to express themselves, especially a personality who is male in a female body, or female in a male body.
Try understanding each other. Try writing to one another, and coming to a compromise, or it really could explode in your face.