Koyuki is a Kishu Inu, Weed's mate and mother of his children.

Koyuki and Weed met while she was searching for her youngest brother Kotetsu and got caught on a trap, from which Weed frees her. She immediately feels attraction towards him and the two meet again when Weed and her brother were fighting with a stray dog. The two began to meet more often, much to Gin's worry, since Koyuki looks identical to Sakura, Weed's mother and Gin thinks Weed wants to be with her because he still longes for his mother. When Gin tells this to Koyuki, she decides she doesn't deserve to be with Weed and the young couple takes a pause. But when Koyuki's owners decide to move and make her a breeding dog, Weed saves her and admits that he loves her for what she is.

Shortly after, Weed leaves to take care of the Russian dogs and Koyuki stays in Ohu, waiting for him. Once Weed is back, she gives birth to their four puppies: Orion, Rigel, Sirius and Bellatrix.

In the sequel, GDW:O she is still alive but Weed gets seriously wounded trying to protect her. She brings him to a human place where they know her until Weed is healed up. She stays there while Weed goes to fight with his sons.