When I began the Sir William series he was a goat and the series was for younger kids. Sir William was the same sort of alien, weird, scruffy character who dropped in and helped people in need. But he was a goat, invisible to some and appearing as a person to others.
Now he is a human person from the planet Fidea. His favorite shoes are slippers and he likes his job as a waste space barge pilot where he gets to play his music, loud, and blow up things floating around the atmosphere just for fun. The Terror Birds, 1st of the series, is now available on Amazon and Audible is coming soon.
Here is one of the original illustrations showing Sir William and Mac meeting the Northwood Under Shrew army. Created by Nicolas Lonprez.