Skull is a criminal. He was working with the Cat Creature to kidnap the Secret Six puppies. It was revealed that he was a spy working for Meadow. Meadow had sent him to capture the Secret Six so she could win the dog contest for her dog. Then he was sent to prison.
Physical Appearance
Personality
He hates dogs, but he loves tinkering with gadgets. He has made "cool spy stuff" for Scar.
What's New, Scooby-Doo?
Season Two
He and his partner Scar had with them a Great Dane that looked exactly like Scooby-Doo (other than being much fiercer and unable to talk) when they kidnapped the Secret Six. They swapped dogs and kidnapped Scooby also to avoid leaving a witness who could describe them. (The other dog was M'Lady Moonbeam in disguise, part of a last-minute enhancement to the plan once Meadow knew Scooby would be involved in the case.) (Homeward Hound)