As an eldest brother he is the most responsible and brave, but also the most aggressive.
Ginga Densetsu Weed
The siblings and their mother were chased by Shōgun's macaque minions and the mother sacrificed herself in order to give her puppies more time to escape. Kōichi promises to take care of his brothers and luckily the puppies bump into Jerome. Even still the puppies soon get kidnapped by Shōgun's minions, but Yukimura saves them. After that, the brothers join Yukimura's pack and they were trained to kill monkeys. However, the training was interrupted by Jerome and Weed and presumably the puppies became friends with the macaques after Shōgun's defeat.