With all the talk of gender and how it can be different from the sex of the body and how it is unchangeable. I'm still pretty sure Rei is gonna chose to stay as a girl, while Eric is probably going to become a guy if/when his heart gets fixed. The dynamics are interesting. I like that despite having two characters who have had their sex changed. They react completely differently.
Eric is a curious dude who sees it as an experiment and since his two bodies are similar there isn't a reason to be too uncomfortable especially when his female body isn't dying and therefore isn't physically painful to be in. He has admitted he's into men, but he is also a man. Identifies as such and acts as such unless it's to fluster or learn. (Or to fuck Arsenio, even then that is because he wants to protect Rei.) His reasons for remaining in a female body are explained and everything he does works within that.
Rei, however, has said she wants to return being a man. Yet there are hints that she's saying these things out of obligation. She wants to do what her brother can't because of his heart condition, she wants to be a man out of obligation. When she talks about her engagement to Lloyd she says, "I want him to be happy, he should marry a real girl" while crying. She wants her friendship with him back but thinks she can't have that back without going back to being a boy. It's also interesting how that line implies she's a fake girl and that she is actually kind of sad about that. Also, the way she experiences guilt about wearing girls clothes despite really liking them growing up and she even goes as far as to willingly try to forget she was ever a guy. It's possible that Rei had a girl heart from the start (I.e she was trans) and then wound up getting that body which suits her better, but she can't enjoy it because she comes from a patriarchal society. A society that puts a lot of weight on her to be a son so she can inherit the family business and hunt dragons for a living with the other menfolk. Even if that's not what she wants to do.
She has also expressed interest in dressing pretty to impress Arsenio.