Zilly is a pilot in the Vulture Squadron. He is a cringing coward, forever running away and having to be fetched by Muttley. Each new plan fills him with dread, and he usually utters his catchphrase, "Ohhh dear! Ohh my!" before retracting his head into his collar like a tortoise retracting into its shell. Dick Dastardly puts up with his cowardice because he is a reasonably competent pilot, and is the only squadron member able to interpret Klunk's gibberish.
He was temporarily turned into a reckless hero via Hypnotherapy, but his new heroism turned out to be even more damaging to the squadron than his usual cowardice, leaving Dastardly no choice but to change him back to normal.