Fiddler's Green is a sentient part of the Dreaming which all travelers (especially sailors) apparently dream of someday finding. It sometimes likes to take human form, and go wandering. It used the alias Gilbert during one of these trips.
Fiddler's Green's most common form is as a kind, portly man who strongly resembles the author G. K. Chesterton, not only in his physical appearance but also in his love of paradoxes, which give Rose Walker headaches (G. K. Chesterton's eclecticism has notably influenced Gaiman.)
Some years after Dream was captured by Roderick Burgess, he decided to assume the form of a man and made his way to the waking world to live like a regular person. It's in The Doll's House arc in which he plays a very important role, helping Rose Walker to find his brother Jed. When Rose's brother Jed went missing, she was warned about the Corinthian by the Furies and decided to traveled to Florida to find her brother. In the house where she was living, was also living Fiddler's Green, he was then known as Gilbert.
When Rose received a call from the detectives she had hired to find his brother, was told he was in Georgia, Gilbert decides to go with her. Somewhere in the travel, they have problems with the car they were traveling in and are forced to spend the night in the closest hotel. in the same hotel there was a serial killer convention being held; the Corinthian - a nightmare who also left the Dreaming - was one of the guests.
Gilbert recognized the Corinthian when they met in an elevator and gets "a bad feeling". He then hands Rose a piece of paper with the word "Morpheus" written on it and told her to call the name if she was in danger while he was away. That night, one of the conventioneers tried to rape her.
Calling Morpheus' name, he showed up and saved her from the killer. When she finally left the hotel, very confused about the events that had just occurred; Gilbert appeared, carrying Rose's brother Jed, whom he found in the Corinthian's car. Jed was then transported to a hospital, where Gilbert visited him. In one of the visits, Gilbert found Matthew, who was also visiting the boy, following Dream's orders. Gilbert recognizes Matthew as a creature from the Dreaming. Matthew tells Gilbert about Rose Walker being a vortex. At the end of the series, it is implied, though not outright stated, that he was "in love, a little" with Rose.
Gilbert and Matthew returned to the Dreaming just as Morpheus is about to take Rose's life, Gilbert pleads that Morpheus take his life instead of hers, but Morpheus tells him that it isn't possible. Gilbert then apologizes and says goodbye to Rose and re-assumes his position as a place in the Dreaming.
He was killed by The Three in The Kindly Ones. The new Dream attempted to resurrect him but the almost-alive Gilbert stated that if he did "my death would have no meaning." Acquiescing, Dream stopped the process.