Hiro is a molosser-type dog officially recognized as a Great Pyrenees, though some sources identify him as a Saint Bernard. He possesses thick, long pale golden fur, reddish-brown ears, and dark markings along his muzzle bridge with small spots near the front. A vertical scar crosses his right eye, a permanent reminder of the attack that killed his father.

Once a loner, Hiro was driven by a desire for revenge against Kamakiri, the Irish Wolfhound responsible for his father’s death. His personality combines bravery with a short temper, and he openly enjoys fighting. In combat, he employs a signature technique of castrating male opponents, believing only true males deserve to retain their testicles. Despite his aggression toward other males, he shows consistent kindness to females and frequently aids those in distress.

His traumatic childhood involved an ambush by Kamakiri and his followers during which Hiro sustained his eye injury and witnessed his father Yukimasa’s death. He escaped thanks to one of his father’s loyal followers, thereafter dedicating his life to vengeance.

Hiro first assisted the Ōu Army after saving the Akita mix Reika using his signature attack. He later provided crucial support to Gin, John, and Akame when they were threatened by the Great Dane Hōgen and his pack. Though captured alongside Gin and John, he escaped imprisonment with help from Hook, Sasuke, Lefty, Tommy, and Roshi, and also helped rescue Reika.

He achieved his revenge in a prolonged and difficult battle against Kamakiri, ultimately castrating him. This victory marked a turning point in Hiro’s life; he evolved from a solitary figure into a more social, integrated member of the Ōu Army, influenced by the loyalty and righteousness he witnessed in others, particularly John.

He later formed a romantic relationship with Reika, with whom he had four sons, and developed a friendly dynamic with the smaller dog Mole, often permitting him to ride on his back.

During the conflict with the Russian army led by Viktor, Hiro joined Weed’s reinforcement unit. He showed impatience with the slower pet dogs in the group but adhered to orders to remain with them. Attacked by the Russian shepherd Aram, who mistook him for the leader, Hiro was aided by Weed and later participated in Viktor’s defeat on the beach.

In the Japanese Alps, Hiro fought hybrid bears but was injured and ordered to remain behind. He disobeyed after GB was severely wounded and requested to be taken to Weed. Thrown off a cliff, he survived the fall and climbed back up, only to discover GB’s death and Weed’s disappearance.

In *Ginga Densetsu Weed: Orion*, Hiro was captured by Kurohabaki Jinemon and imprisoned in a shed alongside other Ōu Army members. He was later released into the mountains with his family.

His final actions occur in *Ginga: The Last Wars*. Ambushed by Monsoon and his bears, Hiro received slashes to his head and shoulder. While carrying the injured George to safety, he was ambushed again. After defeating one bear with help from George and Rocket, he delivered George to Gin and the others. Ordering his children to be strong, he rushed to aid Gin, who was acting as a decoy near a cliff. Ignoring Gin’s command to stay back, Hiro charged the bears and was crushed under Monsoon’s paw. He shielded Gin from a fatal blow, and both were forced over the cliff into raging waters below. Hiro’s body was later found by a young boy fishing, still holding the unconscious Gin. He was buried by Daisuke, who offered flowers and incense in recognition of his heroism.