Booker "The Lone Wolf" is a one-man military campaign. His “work” mainly involves the killing of Sang Soldiers. Many of them. His Signature Weapon of Choice is the HK G36C.
Biography
Aiding the Expendables
Booker: You're gonna need more men if you expect to get out of here alive.—Booker lays out some advice for the group if they want to survive the ordeal
Booker saves the Expendables by killing all the sangs that have cornered the team as well as destroying a tank. After saving the Expendables, Booker gives the team information on Vilain and a village that hate the Sangs. Later he joins the expendables in the airport battle against Vilain, killing hordes of Sangs. After the battle, having just lost his business he leaves with Mr. Church, Trench and Maggie in a helicopter.
Personality
He works alone due to some past personal loss? Maybe because he's just overall better that way? It's unclear why exactly but it is clear that that is the way he likes to work. He has shown respect for Barney Ross and his team, as well as a personal friendship with Ross. Enough to bother to risk his life to save Ross even though he wasn't hired at all.
Behind the Scenes
Booker appears to be directly based on Chuck Norris, Being a stereotypical "Badass", being able to wipe out an entire company battalion of Mercenaries with ease. Booker's character is meant to play justice to Sphaghetti Westerns.
His name is also a reference to two Characters played by Norris. John T. Booker and J.J. "Lone Wolf" McQuade from Good Guys Wear Black and Lone Wolf McQuade respectively.