Jolly Jack Tarr is the loyal sidekick of English Spark Trelawney Thorpe in the popular stories featuring her (and by extension, his) adventures. When Ms. Thorpe finally appears in person in the comic, there is no sign of Tarr. In the course of events, she gets paired up with Gil, and upon encountering the duo, a British soldier that he must be the "new" Tarr, indicating that it's more of a rotating job-title than one guy's actual name.
In the GG side-story The Crown of the Sleeping King, a former(?) occupant of the role is portrayed as a sailor (as fits the name) and his primary function seems to be letting Trelawney exposit at him. He wears a traditional British sailor's uniform which includes bare feet (shoes don't do well in sailing ship rigging), striped shirt, short jacket (tails got caught in rigging and lines) and "slops", loose trousers and hair grown long, caught back in a pigtail.
Notable exclamations employed by that Jack are "Great Honk!" and "Sufferin' sea weasels!"
Possibly Relevant Outside Information Edit
Jack Tar is a common English nickname for "sailor," in particular those serving the British Empire.