If you have concerns, please make an account so I can PM you, I don't want to spoil anything too much. But I'll let you know what I'm trying to do here.
Okay, like in the first chapter, Silvia says "Not exactly. I'm sure you have an idea of what you are now apart of, but it's different from whatever story you learned it from."
This is hinting that the "story/ game" called kingdom hearts is widely known throughout different realms, but as stories, games, plays ECT. With different characters as the main characters, some who exist and some who don't.
Why does Roxas know the protagonist?
Like in kingdom hearts 3 how Sora knows who Verum rex is, and Verum rex knows who Sora is. Through fiction they know who each other are. I don't want to spoil anything so I'll stop here. I thought fellow kingdom hearts fans could catch on to what I was doing with Roxas being there, I guess not. I hop this clears things up.
There are few who catch on, and that just amazes me. But if this story is to hard to follow, I understand if ya drop it.
I hope this clears things, if not, well damn lol. until next time.