Gray Wolf is Tiger's younger brother. He was thought to have been kidnapped and/or killed by Moo, but it is revealed that he was turned evil and became one of Moo's top henchmen, "The Big Bad Four." This haunts Tiger throughout the series until he is eventually forced to confront his brother.
It is shown in flashbacks that Gray Wolf was quite weak and needy as a pup, thus Tiger was especially hard on him to try to toughen him up. As an evil commander, Gray Wolf is cold and downright ruthless, destroying villages and killing both monsters and humans with the aid of his Cabalos army. This makes Tiger feel especially guilty of the way he raised his brother, but it is also revealed that Moo lied to Gray Wolf to sharpen the hate in his heart, saying that Tiger abandoned him. Tiger in fact was mortally wounded trying to save his brother, and his scar is a result of this battle. Gray Wolf is especially harsh to Tiger when they battle, both cheating and ruthlessly attacking him even when he has surrendered.
After Gray Wolf blinds Tiger, Genki tries to defend Tiger, refusing to stand aside. Gray Wolf charges in to kill Genki, so Tiger is forced to put an end to his brother. Sensing his movements, Tiger runs his horn through Gray Wolf's crest, and subsequently, his heart. (This is censored in the US version) Without a crest and fatally wounded, Gray Wolf tearfully reconciles with his brother before he becomes a lost disk.
In season 3, after the rise of the Phoenix, Gray Wolf's disk is restored, and is discovered and unlocked by a girl named Nina, and her Hopper. Finally learning to trust humans, Gray Wolf decides to join them, although he still shows the somewhat aloof and stubborn traits of his older brother. Tiger finds and is reunited with Gray Wolf, though they soon find that they will be rivals in a key battle at the arena, so Tiger coldly disowns his brother. During the battle, it is revealed that Tiger did so because he couldn't let his emotions and the care he had for his younger brother get in the way of battle. Afterwards, they reconcile on a neutral note, Tiger later saying that Gray Wolf will always be his brother.