So we started our adventure from the perilous Sagar forest and according to the map, behind us is the Arthur's bay. So I'm just wondering...did the main character really came from Arthur's bay before receiving the letter?
I have been thinking a lot too about where in the kingdoms our hero grew up. New Anthur seemed like the logical place at first but then I realised that our character doesn't have a single reputation point there yet, which would be strange if he lived their all his life. In fact he doesn't have a single reputation point at all, which removes the option of living in any city on the map I think.
Where did he grew up then? Well, my guess is that he lived in that one hut in the Wyvern mountains. The two corpses are from an older famous adventurer and his wife who took care of the orphan when he was young. That's why there is a demonic skull in the chest, from a demon the adventurer killed in his glory days. Sagar forest isn't too far down from the mountains either, so it would make sense that your character ends up there. And the adventurer could have protected him from the Wyverns before letting him go further on his own so he could have passed the mountains. Since he's living very secluded he wouldn't have gained reputation anywhere. And the training from the adventurer would teach him your starting skills and traits.
Although if this theory is true, that makes the Vengeful Wraiths you encounter at that hut a lot more symbolic. He is fighting the ghosts of his past, the ones who teached him and cared for him but died to some unknown reason. Maybe someday there will come a quest where we can avenge our adoptive parents and let their spirits finally be free.
I think it makes sense he came from the Wyvern Mountains, because that's Wildlands. Which is an important thing to consider because, regardless of the kingdoms you find yourself in, the character always refers to the inhabitants in a way that implies a distinction between the adventurer and them. "You Varsilians", "You Mercians", and so on and so forth.
That being said, the intro says than an inheritance letter summons you to "the capital". Now, if it refers to New Garand as "the capital" instead of "the capital of Varsilia", that can mean you actually come from Varsilia (thus not the Wildlands), or that you simply find yourself in Varsilia at the time.
I don't think you come from anywhere south in Varsilia because the character says he hasnt ever met Varannari the first time we meet Deep-Shadow. So, where is there no Varannari. The Wyvern Mountains would be a good guess.
We could be from New Anthur, I think, just hear me out. We are described in the following way:
"As an adventurer, you are more concerned about your lack of job and your empty purse"
The say I see it, we could have started with minimal New Anthur reputation (what a civilian there usually has), and lost it in out misadventures. At the start, we are laughably weak to the creatures in New Anthur Bay, which would explain why we lost that rep (tried to do a job, ended up biting more than we could chew, lost rep). That failure also explains your lack of job (you're known to be incompetent and unexperienced, so people dont hire you) and your lack of money.
I guess you're always free to imagine it however you please, though, because the vagueness is obviously intended. No matter where you are or who you talk to (Hell, even the lack of knowledge abut ANY PLACE IN THE MAP AT ALL), you are always treated, and act, as a foreigner.
You come from nowhere. Your home is the world.
Unless there is a lore reason otherwise, I'd vote Solliga.
On-the-map so to speak, but not visitable so no "nobody remembers you" / no rep weirdness if you were raised in New Anther. Could easily have taken a small boat down from Solliga to New Anthur and started the walk from there.