Jerome is a German Shepherd originally trained as a guard dog and contract killer by humans. His initial mission was to lead a team to hunt down and eliminate a genetically modified dog known as P4 or Kaibutsu, who had escaped from a scientific institute. During this mission, the target killed all of Jerome's subordinates.

After Kaibutsu's death, Jerome becomes a key ally and strategic advisor to Weed, the son of the renowned leader Gin. He takes on a mentorship role, providing tactical guidance and combat training, though his lethal methods sometimes conflict with Weed's strict moral code against killing.

When a human shoots Jerome in the leg, he chooses to leave the bullet embedded as a permanent reminder and punishment for his past actions as an assassin. The injury continues to cause him pain and imposes a physical limitation in future conflicts.

Jerome's adherence to lethal force leads to a major rift when he kills two of the villain Hōgen's assassins, Lector and Thunder, against Weed's orders to spare them. As a consequence, Weed exiles Jerome from the main group. Jerome accepts this punishment but continues to operate independently, offering protection and assistance to Weed and his allies when needed.

During the final battle against Hōgen, Jerome fights the leader and is severely injured. He is thrown into a raging river, where he nearly drowns while saving Weed by holding the young dog's head above water until rescue arrives. In the anime adaptation, Jerome dies and later appears to Weed as a spirit in a vision. In the manga continuity, he survives the river but is too injured to participate further in the battle and is taken to a human, Daisuke, to recover from his wounds.

Following these events in the manga, Jerome travels to Hokkaidō. There, he is ambushed and captured by a group of invading Russian dogs led by Viktor. A female German Shepherd named Lidiya is tasked with guarding him. Jerome feigns greater injury to deceive his captors and eventually escapes. He later develops a mutual romantic relationship with Lidiya, who defects from Viktor's group.

Jerome rejoins the fight against the invaders and is present when the Ōu army is captured and held in a gorge without supplies. He participates in the subsequent battle and is wounded again. After the conflict concludes, he remains with Lidiya, and they have six puppies together.

In the ongoing manga narrative, Jerome continues to appear. Recent storylines in *Ginga Densetsu: Requiem* depict him losing an eye, indicating his continued involvement in dangerous situations.