Kotetsu is Koyuki's younger brother.

Kotetsu had heard of the Ohu Army and, even if young, decided to join it so, he ran away from home, causing Koyuki to search for him and meet Weed. Kotetsu finds the Ohu Army but is told to return when older, because still too young to become a soldier. Weed decides to go and look for him to impress Koyuki and finds him being beated up by a group of rogue dogs. Kotetsu loses his tail but gets rescued by Weed and reunited with his sister. Kotetsu keeps company to Koyuki when Weed leaves for Hokkaido and it is him who found Koshiro, Joe and their pack of dogs, calling Weed. He did not join the fight against the hybrid bears and went to live in Ohu with his sister at the end.

In GDW:O, Kotetsu survived the catastrophic events that took place in Ohu but got badly burned and wounded by the fire, however surviving. He becomes a spy for the Kurohabaki clan.