What if Sun’s half-human? I mean, the show never talked about his parentage before, and it was stated in the World of Remnant episodes that a Faunus and a human will always produce a Faunus child of that animal species. That could be a reason why he’s so anti-White Fang and doesn’t seem to care about race at all. Since he was born in Vacuo, he probably didn’t face as much discrimination for being half and half. Experiences with the White Fang and other forms of discrimination are likely more recent for him. By the time he came to Mistral, of course, he had already developed his sunny outlook on life. He’s seen how humans and Faunus can interact positively, because he’s the direct result of that.
I dunno. It’ s just a theory. I really hope we learn more about him this volume.