He is a bit timid, cries easily and relies on his stronger and braver brothers.
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. Luckily the puppies bump into Jerome. Mitsuo soon gets kidnapped by Shōgun's minions, but Yukimura saves him. After that, he and his 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.