A Commanding Officer, otherwise known as CO and known in Japan as a Syogun, is the leader of an army for a battle. Each army has its own unique set of commanding officers and more commanding officers are added to each game.
In Advance Wars, the COs were controlled ("advised") by an Advisor, with the advisor being the person playing the game. However, in subsequent Advance Wars games, the Advisor factor was dropped, with the player instead essentially being the CO they were controlling.
List of Commanding Officers
Advance Wars, Black Hole Rising, Dual Strike