Theta Sigma (Regenerators' timeline)

Commentator Theta Sigma was a version of the the Doctor who existed in an alternative timeline where Rassilon failed to finish the Eye of Harmony before his death. Consequently, he never left Gallifrey and became a commentator rather than a renegade Time Lord. He appeared to be in his sixth incarnation. (BFG: Forever)

In the proper timeline, "Theta Sigma" was the Doctor's nickname at the Academy. (DW: The Armageddon Factor, DW: The Happiness Patrol)