Jax is 100% an npc. He is the only one that addresses the viewer. His license plate during the spudsy adventure is Jax.001
Then in the most recent episode he realizes Cain was able to make him vegan despite not having control over their minds.
My theory is that Cain made a copy of him during an adventure and then got them mixed up. Which is why Cain looks so traumatized in episode 2 when talking about keeping track of who is a player and who is an npc.