I'm really surprised no one has mentioned
Rune, of the Thieves Guild. He's genuinely, honestly one of THE nicest NPCs in the damn game. Hands down. He doesn't demand anything of you first as a favor, which then increases his disposition. He's quite nice and personable ALL of the time. Very sweet. The only thing that could be considered a favor - but then technically it
doesn't count as anything because there's currently
no way to complete it as a mission - is the player keeping a lookout for information about the rune rock tied to his personal history. But as I said, it's not even a completable mission in the game (which I think is sad and frustrating), so it's really just conversation. But he's very kind to you pretty much from the moment you land in the Guild. And even if you wind up pissing the Guild off for rule breaking, he's still empathetic toward you (whereas several others are not):
It's a shame you got booted from the Guild... things were going so well.
He's the greatest sweetheart I've come across. And I sure as hell married him in one of my first PC files.