I subscribe to the theory that Sun is a half-Faunus who mostly or only grew up with his Human parent and may or may not have had very many Faunus friends, if any at all. It would definitely explain a lot.

First, and most importantly, it would explain why he has so little knowledge of Faunus history.

It’s not just that he didn’t know the specifics of the White Fang’s founding, it’s also the fact that he seemed like he didn’t know that the White Fang used to be nonviolent. And it’s not like Faunus are the only ones who know that the White Fang didn’t turn violent until 5 years prior to volume 1. Lisa Lavender, a Human reporter, referred to the White Fang as a “once-peaceful organization” in the pilot episode.

In V1E16: Black and White, Sun calls the White Fang “stupid holier-than-thou creeps that use force to get whatever they want”, and when Blake finished telling him the history of the White Fang and why she left, he had this look on his face:

And he suddenly had a more serious tone to his voice.

He also didn’t know why so many Faunus are bitter about being gifted ownership of Menagerie:

Blake: Well, when you try and take an entire species and put them on an island that’s two-thirds desert, yeah, it tends to be cramped.

Sun: What?! You guys have that much more space? Why waste it? A little desert never hurt anyone!

Blake: It’s not like the desert in Vacuo, Sun. The wildlife here is more dangerous than most other regions.

Sun: So, they give us a terrible island, in the corner of Remnant, to try and shut us up.

Blake: That’s a pretty good summary, yeah.

Sun: Huh. Jerks.

Sure, before volume 4, he had never even been to Menagerie (“Never been in Menagerie before! It’ll be a regular journey to the east!”), but that last part of the exchange makes it seem like no one even told him about the Faunus’ view of being given Menagerie.

So, if he’s a half-Faunus whose Faunus parent wasn’t around when he was growing up, it would explain why he lacks knowledge of Faunus history, or at least, history from the Faunus’ point of view. This is especially if he also never had any deep relationships with the Faunus in whatever town he grew up in, in Vacuo. (Remember: “Hey, I may have moved to Mistral, but I grew up in Vacuo.”)

Note that we don’t necessarily know if he grew up in the capital city of Vacuo. He could’ve lived in a village, or he and his Human parent could have moved around a lot for a while. Remember that World of Remnant: Vacuo said “Citizens live in makeshift homes, uprooting and traveling frequently in response to the fleeting resources and occasional attacks from the creatures of Grimm.”

(Side note: Look at Sun and imagine him living his childhood frequently moving to new locations in a desert just to survive. Holy shit. I really want the show to explore that. Did he even go to combat school, or did he just move to Mistral and register at Haven when he turned 17?)

Second, there’s a little something from V2E1: Best Day Ever…

Sun: I know, we were fighting side by side. She was super fast and I threw a banana at the guy, which sounds gross, but it was awesome.

Neptune: Nice!

Sun: Right? And the best part is, she’s a Faunus.

That was Sun describing the events of Black and White to Neptune. Why is the best part of his story the fact that Blake is a Faunus?

Well, if Sun hasn’t had any actual friendships, or even real relationships at all, with any Faunus, then that would explain his excitement about befriending Blake - he finally has a friend who’s like him, a Faunus. He also seemed pretty excited about Menagerie, but that could just be his upbeat, excitable personality.

It would be pretty interesting if Sun is a half-Faunus who grew up with only his Human parent and never had an actual, real relationship with a Faunus until he met Blake.