I'm not entirely sold on the Jax is an NPC/AI theory. Some of the evidence like the Jax plushie first I think is just Glitch recognizing the fandom would latch on to him immediately
His "master of unlocking things" skill does stand out and there's obviously something we don't know about him given the lack of room on his pin card, I'm just not sold on the AI theory
We see him sad for just a moment before Kaufmo's funeral before he catches himself and gets annoyed. I feel like most of his behavior is just an unhealthy coping mechanism for getting trapped in the Circus, not AI. Goose also said on Tumblr that, of everyone, Jax deserves to be in the Circus the most: why would an AI deserve this punishment?
Given the CCK AIs were supposed to be 57x more immersive, that sounds like a big step up, and I dont think an AI as old as Jax would be capable of blending in so much. And Caine should have more control over an AI than a human, right? (But maybe that's where the "rouge" comes into play?)
My initial read on Caine's comment was just that there was a mixup and some horrible accident in the past, not that someone today is an undercover AI, and so far I'm still standing by it. I can see potential theories for a few of them being AIs (and apparently Film Theory wants to do the same for Pomni next), but I don't see any strong evidence to really suggest that so far. Jax is still definitely hiding something, but I don't think its that he's an AI