Jolly Jack Tarr is the loyal sidekick of English Spark Trelawney Thorpe in the popular stories featuring her (and by extention, his) adventures. While Ms. Thorpe probably is a real person (Ardsley Wooster certainly claims as much), Jack may be just a fictional character.
He's 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 by Jack in the story Crown of the Sleeping King are "Great Honk!" and "Sufferin' sea weasels!"