I thought he was a future version of Aether. Dainsleif looked eerily similar to Aether in a sense, and in the Bough Keeper quest, he mentions a traveling companion. I wasn't sure who could be his traveling companion because he didn't take their name at all. But I felt that he may be referring to the Traveler's other sibling. Here's what I think, when the Traveler's other sibling came into the world, they may have needed a guide for the world. We have Paimon from the beginning of the game, but it's unsure how one sibling could wake up whereas the other one was already in the world. They could have made a few introductions. I mean, who would let their sibling journey alone in the world. But that's just my perspective, and I feel that there must be some reason that things took a turn. The encounter with the Unknown god is still something we don't have enough information on because no one knows what her backstory is or for whom she is working. As for the Dainsleif, we get to see his Abyssal powers and how he is on a curse of immortality to watch his people suffer from the destruction he failed to prevent. For now, I can say that we still don't have enough information on Dainsleif because he rarely appears in quests, and so far, only three quests have featured him. We have so many regions and areas launched in that time, and I speculate that we might see him Sumeru is a key quest.