Gustav Leatherback has had many different jobs. He was a bodyguard for Zhang, and is currently Brittney Barker's trainer. But before then, he was just a common criminal. He was abandoned back in his native Australia, and was forced to do whatever it took to survive. As a teenager, he was brought in by a Karate master in order to control his rage.

In the years that went by, Gustav only seemed to get in more fights, and his body is the sign of his hardships. His muscles are the result of the brutal training he endured in order to survive. The scars across those muscles are the remenants of various battles. He made his mark in one of Zhang's tournaments, becoming the champion after killing his opponent in the final round. He retained this title in the next two tournaments, but in his fourth, he was dethroned by Kentaro.

After that, he returned to Australia, and decided to bury his criminal past forever. Now working to train the daughter of his new boss, Ted Barker, Gustav will often show his "results" when he spars with Brittney, to teach her the hardships that one has when the follow the fighter's path.