Jerome is one of the dogs specially trained to be a security dog at the lab where Kaibutsu was experimented. For five years he would talk through a vent to encouage Kaibutsu to get through the pain from experimentation, he said it would be worthit in the end. When Kaibutsu could no longer take the pain, he escapes the lab. Jerome and the other guard dogs are sent to find and kill Kaibutsu. He and his assassin friends (North, Rocca, Heuler and Robert) meet Weed and his pack. The dogs of Ou show their strength and spirit by jumping over the canyon near Gajou. Jerome's group decides to work with Weed's group to take down Kaibutsu since he was a common enemy. North shows Weed the way in Gajou and they plan to lure Kaibutsu out for an ambush. North is killed but this doesn't shake Jerome's nerve or the other Assasins. Smith sacrifices his life to make rocks fall on Kaibutsu.

Later the next day, Jerome and Kaibutsu have their last talk. Kaibutsu doesn't want to kill Jerome because he owes him those years of comforting words while in the lab. Jerome tries to explain a dog should be loyal to it's master no matter what they put their dog through, it's a dogs duty to be faithful. Since ancient times dog and man worked side by side. The words mean nothing, Kaibutsu has truly become a monster. Jerome' remaining subordinates attack Kaibutsu's weak point in the chest. They all fall in the line of duty and Jerome is nearly suffocated when he attacks Kaibutsu. He gets the monster's heart in his jaws. Kaibutsu gets pushed off the cliff and impaled on a branch below thanks to Weed. Jerome survives the fall and decides to live in the wild and serve Ou.

Jerome becomes Weed's advisor and keeps a close on Weed at all times. He teaches Weed how to fight. Jerome is good at judging character and could tell Rocket was one of Hougen's men. Weed gets captured by Rocket and his Borzoi brothers but Jerome comes to the rescue and defeats them. After Rocket realizes the good spirit that Ou soldiers have, he tires to join to be a real member of the pack. Jerome doesn't trust him after what happened even though Weed said he trusts Rocket to lead them throught  the Alps to gather more comrades to fight Hougen. When Rocket risks his life to save Weed and GB from falling off a cliff, Jeromes rescues Rocket and tells him he can lead the way.

When the Ou dogs are being shot at by hunters, Jerome jumps in the way and takes a bullet for Weed. His life is dedicated to his leader. Later on Jerome kills Thunder and Lecter which angers Weed. Weed did not want him to kill them. He gets exiled by Weed and all the other dogs get greatly stirred by this. As an exile Jerome gains followers and will always remain faithful to Weed.

Later on Hougen pushes Weed in to an ice cold river covered by ice. Jerome sacrifices his life to save Weed.

Unlike in the anime, Jerome does not die in manga. He reunites with Weed and aids the Ou army.

Jerome is captured Russian army dogs that attacked Hakuro's terirory in Hokkaido, but he presuades his guard, a female German Shepherd named Lydia to let him go and join Ou army. Lydia becomes Jerome's mate later on and in Ginga Densetsu Weed: Orion they they have six pups. Four are black and white like Jerome and two are blue like Lydia.