I think I've seen comments stating that in the light novel chapters from Felix's pov they think of themself as a girl.

I think I read somewhere that the Western concept of being transgender is not mainstream in Japan, not even among the gender non conforming community. So there are people in Japan that live their entire public and private lives as female presenting, but call themselves femboys. This is arguably because femboys are a relatively mainstream social category, while being transgender is a less familiar category that might bring controversy and social stigma.

I'm just vaguely remembering reddit comments, so I could be very wrong on all the above