Lieutenant Hugo Lang was an officer in the Interplanetary Pursuit Squadron. He was sent to find the abductors of Romulus and Remus Sylvest, but was shot down over Titan III and crashed. He was rescued by the Sixth Doctor and travelled with him to Jaconda, where he helped him defeat Mestor. Afterwards, as he had no family and nothing to go back to Earth for, he decided to stay and help the Jacondans rebuild their world. (DW: The Twin Dilemma)
The Sixth Doctor's companions from stage plays are difficult to assert with certainty, as different actors played the Doctor in the same play. If a medium is not mentioned, then this incarnation did not have companions who were original to that medium; it does not mean that this Doctor failed to appear in that medium.
