ThetaSigma

In July 2016, the Twelfth Doctor joined an internet forum under the name ThetaSigma. He wrote an extremely negative post about an episode of a television series. (COMIC: Planet of the Rude) He had previously been known by the name at the Time Lord Academy. (TV: The Happiness Patrol et al.)

Behind the scenes
The Doctor's name is in reference to the Doctor's nickname while at the Time Lord Academy, Theta Sigma.