Nobuo is a young orphan boy, living in Uwajima in Shikoku.
It's unknown what happened to his parents.
Ginga Nagareboshi Gin
Nobuo is an orphan, probably has been since very little.
When he was around 8 years of age, he met wandering Benizakura. He took the dog in, offering him home and food and calling him "Don". Benizakura started to gain money in bull fighting to show gratitude and to take care of the orphan boy.
This made Nobuo believe that he can live on his own with Benizakura and does so for whole 4 years. His uncle constantly tries to offer him help and home, but the boy refuses, thinking that all he needs is Benizakura.
After Benizakura carries unconscious Cross to him, he decides to take care of her. When Gin and Smith show up, Nobuo assumes they're Benizakura's friends and decides to offer them food. He is disappointed to find out that the dogs have left when he comes with the food.
Later, Nobuo's uncle tries to talk some sense to him, but the boy doesn't listen. Angrily, the uncle hits and shouts at the boy but even still Nobuo doesn't listen. Finally, Benizakura realizes that he had made the boy grow dependent of him and leaves the boy, thinking his well being and his own new mission.
It is unknown what happened to Nobuo after this. Likely he started to live with his uncle and missed Benizakura greatly.
Nobuo is first seen when Benizakura is fighting a bull and when Gin and Wilson show up.
He is seen crying when Benizakura leaves, reminding Gin of Daisuke.
- Nobuo's backstory isn't explained at all in the anime and there's no mention of him being an orphan.