Sirius came from a planet in the Dog Star to save the canines of Earth from the threat of nuclear war. He contacted Cadpig and seems to have a crush on her.
Sirius seems to have various abilities, such as the ability to grant the power of flight.
His true appearance is unknown, as whoever is looking at him sees him as their own breed. He also believes that humans are the owners, with the dogs as the pets.
Sirius told Cadpig, Pongo, and Missis to gather all the dogs of London to Central Park. Once all the dogs came to the park, Sirius revealed that he put all the humans on Earth into a deep sleep, and invited them to his planet to get away from a nuclear war. The dogs asked Cadpig and Pongo about this and they told them that they were happy on Earth. When Sirius heard this, he understood and went back to his home planet. He was never seen or heard from again after that.