He is a large fish who wears a white buttoned coat, light blue glasses, and a light blue head mirror.
In most episodes, he is purple with light blue lips. In "Once Bitten," he is light orange with light green lips. In the book Behold, No Cavities!, he is lime green with dark green lips. Just like Perch Perkins, who also has purple and orange forms, the two colors of him are the same character model.
He is a snail veterinarian in his first appearance, "I Was a Teenage Gary," where he tells SpongeBob that in order to cure Gary, he must inject him with Snail Plasma because he was "too squeamish."
He makes a brief appearance in "Band Geeks," coming to Squidward's house from the animal hospital. He thinks that there is a dying animal on Squidward's premises.
In "Once Bitten," he appears when everyone panics in the Krusty Krab about having Mad Snail Disease. He then explains to everyone that he is a S.D.E. and S.E. (snail disease expert and snail expert). Then he tells them that there is no such thing as Mad Snail Disease and that it is just a myth. To prove his point, he takes a "small" splinter out of Gary's foot, thereby curing Grouchy Snailitis.
This is his most common job. He appears in "Suds," where he assigns treatment to SpongeBob while he has the suds.
In "Squid's Day Off," he helps reattach Mr. Krabs' arms after they fell off.
In "The Bully," he tells Flats the Flounder that he was in the hospital and that SpongeBob had performed CPR on him for five hours straight.
In "No Weenies Allowed," he recommends that SpongeBob go to the Weenie Hut General Hospital, rather than the hospital he was in.
He performs 20 hour-surgery on SpongeBob's butt by repairing it with staples and glue in "I Had an Accident."
He makes a brief cameo in "SquidBob TentaclePants," where he tells a woman that she had given birth to a healthy baby, only to have the fused SpongeBob and Squidward to teleport in the baby's place as he shows it to the woman.
In "The Two Faces of Squidward," he performs Squidward's operation. SpongeBob's interruptions make him feel paranoid during the final stages of the operation.
On July 26, 2019, a tweet from the official SpongeBob Twitter account mentioned him. The tweet was captioned "just go see Dr. Gill Gilliam." It was a joke about how looking up symptoms online can make people paranoid.