So, nobody is gonna say who our boy, Dain, is? Oh, cool, I will have to lay it down on you.
1. We can see that Dain is from the same place as Kaeya, Kaenri’ah, which is known as a place that is godless. (plus, Dain and Kaeya have the same shape of irises)
2. You can see that he does not have a vision but he has a particular hate for the abyss.
3. He knows every archon (at least all that we have seen until now).
4.He is far older than he seems.
5. He traveled with your sibling.
Let’s put them head to head. The Dainsleif is the archon of Kaenri’ah who went on a journey with the traveler’s sibling a lot of time before, before the Traveler came to Teyvat. When he came back to Kaenri’ah, the place has already been destroyed by Gold, the alchimist(his home being destroyed is eluded to when Paimon and the Traveler speak about settling down after finding the Sibling). He doesn’t have a vision, but he obviously has powers, which can only mean that he either has a gnosis or is an adepti. We know that Kaenri’ah comes after Snezhnaya, and there we find him again(in the trailers).