He is the last leader of Black Wolf's Empire and the ruler of the Underworld.
Gaia is a big black-and-white Hokkaido Wolf. He has a slashed right eye and it's unknown how he got it. He has black grin-like markings around his mouth and dark grey face. He has fluffy white cheeks and white eyebrows. He has very sharp and strong fangs he uses to cut limbs and heads from his opponents.
Gaia is a cruel, evil wolf lord who appears to be more or less insane. He is extremely self-centered. He views himself as a superior and god-like figure, refusing to give his rule to anyone and thinking that his word is the law. Gaia doesn't listen to explanations and kills all minions who fail to complete their mission.He drives to kill anyone who challenges him and shows no mercy. He doesn't care about his soldiers and only sees them as tools, Pup Soldiers too. He seems to be very manipulative towards the young wolves of his pack, raising them to killers who serve him with their lives without questions. Gaia believes that the strong have right to rule and that the rulers will decide what is right and what is wrong.
Gaia was born in the Underworld kingdom in Hokkaido. He is possibly the son of Purple Wolf, which would make him the tenth-generation descendant of Black Wolf. His kin was mortal enemies with Reima's and took over the Underworld kingdom from them. Gaia apparently became the Emperor of the Underworld after his father, during the 10 years when Reima was away. He was both greatly respected and feared by his minions. He was viewed insane, evil and had even god-like status among his followers. Gaia believes that he is the true leader of wolf kind and only one who deserves to rule the Underworld. He trains 8 special wolves to become the legendary hakkenshi of Sirius. After hearing that there is a dog who masters Zetsu Tenrō Battōga, he orders Mugen and his Ten Fighters Of The Underworld to find him and Reima's secret Palace. The minions succeed but get defeated.
Reima swims to Hokkaidō to face Gaia's Empire. After his 8 warriors and commander Juga get defeated, Gaia fights himself and refuses to give up even when all his soldiers die. He is so strong that he is capable of fighting several soldiers of Gin and Reima alone. He rips off Reima's leg and easily defeats Ben, Hyōma, Kurotora, Retsuga and John. He kills Raiga, Sakon and Ukon by ripping their heads off with his deadly fangs. However, he too gets hurt in the process - He loses his ear. Akame loses the fight against him but manages to wound him and cut off one of his fangs. Gaia is great opponent and he constantly tries to kill Gin who he views as his most dangerous opponent. Finally, he starts to get tired and his head is badly wounded by Gin's Battōga. In rage, Gin almost kills Gaia when suddenly an earthquake breaks loose and causes a landslide. Gaia sees his final chance and almost catches Gin but is taken aside by Reima. Unable to leave the collapsing kingdom, the two wolf lords face each other in the final fight. Gaia is eventually crushed by heavy rocks and dies along with his Empire.