Earl Sigma

Earl Sigma was a blues player from Earth who played blues songs on Terra Alpha. However this style of music was deemed "unhappy" and outlawed, leaving him with no means of income to buy a way off planet, stranding him there. He helped the Seventh Doctor through the Kandy Man's kitchen, snipers and loads of other dangers. After the Happiness Patrol and Helen A were deposed, Earl decided to stay and teach the people there the blues again. (TV: The Happiness Patrol)

Behind the scenes
Sigma was not Earl's last name, but a designation given by Terra Alpha to people from other planets.