Fuji is a female Akita belonging to Daisuke and his familly. She gives birth to Riki's pups - Gin and his two borthers. Gin is taken away from her by Takeda Gohei to be trained as a bearhound. She loves her pups but allows Gin to make his own decisions including joining the wild dog pack.
The manga mentions that she used to be a hunting dog untill she retired to give birth to her pups.