The Time Sentinel was a singular entity responsible for maintaining the scribbled timelines on the Axis. Its many aspects are also colloqually known as the Time Sentinels. Aspects had rounded heads, and their skin had various colours, including blue, red, white, (COMIC: Old Girl) and black. (COMIC: The Good Companion, The Tides of Time, et. al.) They were able to display images and symbols on their heads, including several incarnations of the Doctor and his TARDIS.
The Aspects of the Time Sentinel were present on prehistoric Gallifrey, where they threw time sensitives into the Untempered Schism. (COMIC: Old Girl)
The Time Sentinel 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. They are guided by the Lock Keeper, an aspect who is the cardinal guardian of the time lock. (COMIC: The Good Companion)
The Doctor stated that he had seen something like them a long time ago, and he had believed that they were Matrix agents. (COMIC: Old Girl)