Barrett is the leader of Avalanche, he fights against Shinra to try to save the planet from their Mako sucking reactors. Barrett relies on brute force and is easily the strongest character
Barrett's gun arm allows him to reach flying enemies and attack at long range without the usual decrease in damage that befalls most of the other characters. Barrett is not particularly good with magic though so don't rely on him for healing, leave that to Aeris/Tifa. Barrett's best weapon is the Missing Score, which will increase in power with the more materia you equip on it. His best limit break is Catastrophe..