Who is the guy in the DotA 2 Trailer?
Solution 1:
In all likelyhood, he is the shopkeeper for the store.
DotA has no real plot to speak of.
Solution 2:
He is indeed the Shop Keeper of Dota 2 Secret Store.
You can learn more about him by reading Tales From Secret The Shop - Are We Heroes Yet?, a comic published by Valve.
(source: steampowered.com)