If the Jax npc theory was real, I would want it to happen like this:
Jax knows he is an npc, the others do not, Caine can't tell the difference and has forgotten
Jax has spent all this time trying to make sure Caine doesn't notice the fact that he is an npc
The actual reveal: it happens just before they're sent into another game, everyone is gathered up together, Jax is talking to Pomni about something stupid, arguing.
Caine is arguing with Zooble, not paying attention. Jax says something harmless and throwaway in the background. Zooble starts to answer Caine back.
Camera zooms out behind Caine, to give more of a perspective of Jax and Pomni. The lighting darkens slightly.
Caine's head spins 180° to face Jax, who freezes mid joke.
Jax doesn't look away from Pomni, but he lets our a small breath.
And then he turns tail and runs