Time Sentinels were responsible for maintaining the scribbled timelines on the Axis. They had rounded heads, and their skin had various colours, blue, red and white among them. They were able to display images and symbols on their heads, including several incarnations of the Doctor and his TARDIS. They were present on prehistoric Gallifrey, where they threw time sensitives into the Untempered Schism.
The Time Sentinels later engaged in a kind of civil war caused by the rogue Aspect Red's attempt to take control of the rest. During this event, Aspect Blue referred to a now departed former lone agent, "Aspect Black". (COMIC: The Good Companion)