Hachi is presumably the previous Commander-in-Chief of the Orange Star, and is currently retired, owning the Battle Maps section in the first three installments.
He is rumored to be Orange Star's former chief, and is stated in the game to be a former rival of Sensei. He is also the keeper of the Battlemaps shop in Advance Wars, Advance Wars 2: Black Hole Rising, and Advance Wars: Dual Strike. He gets a discount on all units, but unlike Colin's, his units have normal firepower. He has no real weaknesses. He is also extremely gossipy and may give you hints about the game in exchange for you buying his merchandise. If you clean him out, though, he won't give you anything.