Who is the guy in the DotA 2 Trailer?

In all likelyhood, he is the shopkeeper for the store.

DotA has no real plot to speak of.

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)}