The Time Sentinel was a singular entity responsible for maintaining the scribbled timelines on the Axis. Its many aspects are also colloquially 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 Time Sentinel was a system left behind by the Time Lords to monitor the timelines. Based on an asteroid with its own Eye of Harmony, its various aspects were denoted by colour. (COMIC: The Good Companion)
The Aspects of the Time Sentinel were present on prehistoric Gallifrey, where they threw time sensitives into the Untempered Schism. After crossing paths with the Tenth Doctor, the Time Lord told Cindy Wu that he had seen something like them a long time ago and he had believed that they were Matrix agents. (COMIC: Old Girl)
Worried that such acts by the Doctor could damage the time lock surrounding the Last Great Time War, the Time Sentinels recruited the Red TARDIS to abduct Gabby and attempted to trap the Doctor in an alternate timeline that they would seal off. When the Doctor refused to take the bait however, Aspect Red ordered an open attack on Aramuko before Aspect Blue, convinced that the Doctor was not a threat, disabled him and tried to make peace with the Doctor. When the Doctor materialised on the Sentinel's asteroid, the Red TARDIS revealed itself as the true mastermind and corrupted all the Sentinels, save Aspect Blue, turning them against the Doctor. As the asteroid base was leeching power from the Osiran Circle of Transcendence, Noobis caused it to collapse, destroying all of the Time Sentinels. (COMIC: The Good Companion)