Theory, don't need a confirmation:
Spoilers Below
Eternum is far more than just any full dive VR game, it's a realm of reality, I think it's a world outside the real world, and someone's goal in Ulysese's is to have everyone live the lives forever in Eternum, I think the NPC's in eternum could be the souls of Dead Humans, Either living out their lives, completing their goals, or ultimately trying to find their place.
Thanatos is a good example of this, from where I am in the game currently, I've seen that Thanatos might actually be dead, and how he is practically Ulysese's way of getting the Gem of Doom (Mainly William Bardot, he's their mercenary, that they can send where ever he is needed, which explains how he got on to the unreleased server. He probably was a high level player, and the company most likely saw use in him, so he died, maybe by a praetorian or some other force, so then they could make him a realistic NPC that they can use to track down the Gem of Doom to overthrow the Founder.
I think that NPC's are dead people, my only proof to back this up was when MC was getting all the flashbacks, when he saw a picture of the Woman's son, Thanatos is a Human that died, and became a NPC, which could mean that Eternum could be a whole different world/Reality, that is controlled by the company, and the NPC's are just dead trapped souls
Kind of like the creator of SAO trapping people in his game, so that he could experience what it was like to own his own world, and take the role of God.
This is all I have to say for now, but this game has some insane lore in it. Can't wait to discover more