Kōshirō is a main character of Ginga Densetsu Weed in the Hybrid Arc and a minor character of Ginga Densetsu Weed: Orion, having just one single appearance in the first chapter.
He is the great-great-grandson of Shiro and Yamabuki, the great-grandson of Riki and Fuji, the grandson of Gin and Sakura, the son of Joe and Hitomi, the nephew of Weed and Yukimura, the brother of his 3 deceased siblings, the cousin of Bellatrix, Orion, Rigel and Sirius, and the second-cousin of Shion.
Kōshirō has greenish-brown eyes and pale red fur.
Like his uncle Weed, he is calm and polite and very excited to know that he has other family members.
He was born in the mountains along with his three brothers, but his mother Hitomi and his brothers were killed by the Hybrid Grizzly Bear.
Joe realized that one of the puppies survived and named him "the happy fourth son", so he could live happy, not just for himself, but for his brothers.
Just like his father Joe, Kōshirō became Ninomiya's pet and was called "Chibi" by him.
He later met Koyuki and Kotetsu while hunting. Kōshirō also met his uncle, Weed, but Joe didn't want to be in any terms with him and beat up his son when he went to see him.
Later, Kōshirō was taken to safety during the battle against the hybrid bears. After the battle was over, Kōshirō went to live in Ōu with his father.
When the volcanic eruption destroys Ōu, Kōshirō is seen fleeing with the other Ōu soldiers.
After that, he is not seen or mentioned in this series, as such it is unknown if he survived the volcano eruption or not.
- Kōshirō's status is unknown, as it was never stated if he survived the volcano eruption in Ginga Densetsu Weed: Orion or not. He was just not seen anymore after that.
- When looking at the first coloured images of his great-great-grandfather Shiro, Kōshirō resamble his fur colour.