Doc is an inventor and the only regular human character in Fraggle Rock (as well as the only character who appeared in every single episode). He lives with his dog, and best friend, Sprocket. Doc spends a lot of his time in his workshop, a refurbished old room which is directly connected to the caves of Fraggle Rock via a hole in the wall.
As a human, Doc is considered one of the "silly creatures" by the Fraggles. He often lives up to this designation with his sometimes impractical schemes and inventions, but he is always enthusiastic about his work, and is a good friend to Sprocket. Doc's accomplishments include such varied inventions as the Rocket Sprocket and the Collapsible Bookcase, and being named Man of the Year by the North American Society of Tinkerers. Prior to adopting inventing and the pursuit of scientific advancement as his life's work, Doc was employed as a barber. He has an off-and-on friendship/rivalry with next door neighbor Ned Shimmelfinny.
Whenever Uncle Traveling Matt sends postcards to Gobo, they are delivered to Doc who promptly throws them away. This makes it necessary for Gobo to venture into Doc's workshop to retrieve the postcards. Doc is continually puzzled by the missives for this "Gobo Fraggle fella."
In the second season episode "A Cave of One's Own", the arrival of a new neighbor, Ms. Ardath, provides a romantic interest for Doc and a new sideline, as part-time manager of The Captain's Inn, a bed-and-breakfast that was built across the street.
In the second to last episode, "The Honk of Honks", Doc finally learns of the existence of Gobo. However, in "Change of Address," Doc and Sprocket have to move away to the desert a considerable distance away to be with Ned for his health, although both parties soon learn that you cannot leave the magic.
Doc also appeared in A Muppet Family Christmas, accompanied by Sprocket having rented Emily Bear's cottage at Grizzly Farm for Christmas until Fozzie Bear arrives with his friends to spend Christmas with his mother. Though initially taken aback by the sudden arrival of the Muppets, Doc adjusts and befriends the motley group even going so far as to assist Kermit the Frog in rescuing Miss Piggy from the blizzard. Doc even joins in on the Christmas melodies and even plays Santa Claus for the troupe. (While this special aired after the Fraggle Rock series had ended, it apparently takes place during the period prior to Doc's discovery of the Fraggles' existence; at one point he even asks Sprocket if the Muppets are "anything like these Fraggles you keep telling me about".)
In the final episode, "Change of Address", Doc tells Gobo that his real name is Jerome Crystal. As Henson staff writer Craig Shemin pointed out at the 2001 MuppetFest convention, this would make him "Doc Crystal."