The characteristics of Bill, including physical appearance and voice acting.
Bill is a Dog Soldier like most other members in the Cornerian military. He could resemble a few different breeds of dog, including a Labrador, a terrier, or a bulldog; however, most choose to label him as a bulldog. Bill is most likely an Old English Bulldog or an American Bulldog due to his longer muzzle and his slimmer body than an English or French Bulldog.
Bill's fur color changed between his two appearances in Star Fox 64 and Star Fox Command. In the former, he actually was a shade of grey, befitting his name. In the latter, he was a light brown. He wears a Green Ace Army Commander Helmet on his head because he's a Commander of his own team. Bill's eye colour went unseen until the character redesign in Star Fox Command as they were hidden behind his helmet, they are now green.
He speaks only Lylat Language in Star Fox Command like the other characters. Also, for unknown reasons, his voice (or more accurately, the text transcription representing his voice) was changed from a surfer accent to a southern accent (although he retains the surfer accent in Star Fox 64 3D).
Since many elements from Star Fox 2 were carried over to Star Fox 64, it's largely speculated that Bill was created as an equivalent or a replacement for Fay.