He is called Drakov in the European release (Advance Wars: Dark Conflict)
The Beast is a former Rubinelle soldier, who after the Meteor strikes, leads a group of bandits to find whatever they can to survive, even if it means stealing from others. He serves as the games early antagonist, first leading a small group of bandits, that attempts to raid multiple communities, until they're ultimate defeat at the hands of 12th Battalion, with only him surviving. Then Caulder gives him supplies and soldiers to fight against 12th Battalion, but he ultimately fails Caulder's test, and ends up getting killed.
The Beast is a ruthless sociopath with no value for life or morality. He seeks only that which furthers his own selfish cause.