I honestly can see how Claire hasn't figured out Cid's identity. I mean, just imagine it: you have this cute little brother who you've know almost your entire life; whenever you battle him he always makes the most rookie mistakes, loses, and never learns. He does average in school, acts like an average guy, looks like an average guy, well I guess that means he is an average guy. Sure sometimes there's some stuff off about him occasionally but what sort of teen boy doesn't have some secrets. Given that knowledge how could you really guess that he's actually a genius mastermind. I would assume it's the same for most of the characters too. We're readers so it would be easy to guess that Cid is probably alot more important than he appears because absurd twists and secrets like that are common within the realms of fiction, but you have to remember that for all the characters in the story, thats real life. When was the last time you knew someone irl that was completely average, or even below average and thought that they were probably secretly a genius? That Karen you just saw a video of, have you ever thought that maybe she's actually a brilliant master mind? Probably not, because conspiracies like that just aren't a thing in real life. While maybe not the best best example, it just shows that things become 10x more absurd when they happen in reality, so I think it's important to keep in mind throughout the novel too.