Trevor Sigma

Trevor Sigma worked as the official Galactic Censor. The "Sigma" designated that he was from a planet other than Terra Alpha. Trevor was a bureaucrat sent to identify those citizens of Terra Alpha who had disappeared since his last census of six months' previously. His work on the planet coincided with the Seventh Doctor's successful attempt to remove Helen A from power by starting a revolution.

Biography from the TARDIS Data Core article, licensed under CC-BY-SA