He originally met Kinumaru while she was trying to drink from a swimming pool, and helped her survive as a stray. They eventually had 9 pups together, and he became the sole provider to his new family.
One day, Billy mysteriously disappeared, leaving his family to fend for themselves - 8 of the pups died, eventually leaving only his son Shion alive.
There is no information on what caused Billy's sudden absence because there is no conclusive evidence related to where he is and if he is living or deceased. Because of his disapperance Billy is shown very briefly but him not being there influences the whole plot of the story because his absence inadvertently led to the death of all but one of his pups, and later the death of his mate from her illness.