Barret is the short tempered leader of Avalanche. His "daughter" Marlene is his only remaining loved one. He used to live in Corel when it was a prosperous mining town, but he wanted the town to move into a more modern age by using mako for their energy. When an accident occured at the Corel reactor, Shinra took it upon themselves to take revenge. The whole town was burned down and Barret blamed it all on himself. All that is left is a sick impression of Corels past greatness. Marlene isn't actually his daughter, she is Dynes daughter. Dyne was Barrets old friend, who has a machine gun on his left arm. Dyne wants to kill everyone and everything, even Marlene, so Barret has the sad job of disposing of him.
Barret is an offensive attacker. His machine gun is good for fighting some bosses that are out of reach, like that elevator boss on your first trip to Shinra headquarters. Not much of a magic user, but if sent into an unavoidable situation, he could easily adapt to magic.