My personal nerdanel, as I believe you call it here, is that the traveler's identity is largely irrelevant compared to his purpose.
I always loved my namesake and he's definitely my favourite character. I got to wondering though at some point...just what is Kosoter? The first theory I had was just that he was simply Palatine (I think?) Soter because he was a noble Veteran from the Holy War that definitely was on Team Kellhus. Then I wondered if he was one of Nascenti but they would all have powerful positions in the Ministrate and not be out killing sranc for a living. I've concluded that he was indeed just a fanatical warrior from the Unification Wars that became famous but the blasted and evil things Mimara sees in him with her Judging Eye speaks of something far darker and terrible than anything else. Achamian is clearly going to be pretty damned, certainly moreso than the Skin Eaters, whatever their histories. Surely Kosoter can't just be that much more of a dick than them as far as the gods are concerned? There had to be some other answers.
I think the scene that really did it for me was when he looked at all the fire and horror of Moria 2.0 and everyone else thought they'd come to the outer precincts of Hell. He dismissed it because "if it was Hell, I would remember". At first this simply sounded like some token badass comment but Kosoter doesn't just go around making badass comments. He isn't Cnaiür. Instead he makes a fairly cryptic comment about Hell and it dawned on me later that maybe he is telling the truth...or thereabouts. Maybe Kellhus did indeed make his own Inverse Fire and show it to a bunch of people to utterly condition them to his cause? That was the main reason I thought the traveler came to get Kosoter...to simply show him the IF and it's why he is so utterly dedicated to 'saving our souls' with the Princess. He's not just some local guide and guardian that they chose at random- there were other Veterans and they'd easily be convinced by enough gold, power or 'FOR THE EMPRAH' shtick. Kellhus could have found a way. Instead, he chose Kosoter, a man that has been going on slogs since they became a thing. Why? He must have enough wealth or influence to be somewhere else? It can't simply be devotion to Kellhus that led him to do what he did and what no other men really could do. Kellhus intended for Mimara to be kept safe for a reason. He had a plan for her and Achamian).
The total crackpot I've also been kicking around about it is the one off-reference to walking the Outside. If someone goes there, they might return possessed by a ciphrang. Kellhus went around with the heads of them so he's clearly done it and knows how from the Daimos he's been learning. I thought this a simple misdirection that you can't see Kellhus' POV not only because it'd give away everything but because he's now controlled by a ciphrang. A ciphrang is given a task by its master when it is forced into the world and it isn't released until it's done it's job. It will do ANYTHING to accomplish it. I think it's possible that Kosoter was taken away by the traveler and possessed by a ciphrang and that's why he simply stands there when he's taken Akka and Mimara to the library. He just sits there, waiting to die. Relieved, almost. Relieved of all his burdens and free to return to the Outside like a good little ciphrang. I definitely think that's a far out possibility but it's one I've been considering for a long time.
Still though, far more likely for me that the traveler is just a dude that came up to show Kosoter the IF and convince him to protect Mimara and Akka until they get to the library. He must have been convinced that she has the power to save their souls. With his task complete, he accepts his fate. With his reference to remembering Hell (actual Hell, not simply terrible conditions from the war) and how utterly evil he looked in the Judging Eye, there's no way he can just be a simple dude.