Gabriel and Tanith

Gabriel and Tanith were two beings indirectly created as the embodiments of the suffering of the universe, who "generated entropy" and caused death merely because they could and sadistically enjoyed themselves. (PROSE: Falls the Shadow) They embodied the people who would never exist, whose lives were snuffed out by time travel. (PROSE: Head Games)

Initially appearing as balls of lights, Gabriel and Tanith created their bodies using pieces from the many people they killed, until they became beautiful and flawless human beings. They acted like brother and sister, but sometimes kissed and behaved differently. Anything done to them, such as being shot, would let them be "reborn" by the experience; their bodies and clothes repaired themselves and they were essentially immortal.

They were capable of doing anything as they drew their power from the structure of reality itself. This included flight during their battle with the Grey Man, pulling individuals from alternate timelines, and making the Doctor's TARDIS perform calculations for them. Gabriel later tortured Jane Page until she became blind and Tanith enabled Jeremy Winterdawn to walk again, the two of them even killing Bernice Summerfield, sending her soul to Cathedral. They also knew of Ace's arrival on Svartos and questioned her as to the nature of time travel, explaining that every time-traveller was responsible for countless genocides: "Every time you blink, you negate all futures in which you didn't."

Eventually, on Cathedral, they abolished/are abolishing/would abolish time and space, leaving Cathedral to be their kingdom from which they would rule, making both everything and nothing mean anything. However, their power was cut off and they were turned mortal. Ace immediately stabbed Gabriel and killed him, before chasing down Tanith and fatally shooting her. (PROSE: Falls the Shadow)

When later explaining the Miracle as a breach between N-Space and a fictional sub-dimension, the Seventh Doctor suggested that Gabriel and Tanith created it as a by-product of their existence, although he knew in truth the breach was created by Kadiatu Lethbridge-Stewart's time travel. As he tried to seal off the Miracle, the fictional energy confronted the Doctor in the form of his predecessor, who questioned his actions as Time's Champion:

"And the ones you’ve killed — the people that you’ve decided shouldn’t live on in the universe that you’re creating what about them? What about Gabriel and Tanith?"

- PROSE: Head Games

Later at her wedding, Benny received a letter from them which caused her to laugh as the Doctor read: "Much joy. Hope all goes well. Why didn’t you invite us? Love, Tanith and Gabriel." When Jason arrived, Benny complained at he'd killed her twice, beating Gabriel and Tanith's record. (PROSE: Happy Endings)

As a result of their actions, the universe would now end "several metacenturies" earlier than projected. (PROSE: Falls the Shadow)

Behind the scenes
Although the cover art depicts Tanith with a short haircut, the novel describes her as possessing shoulder-length hair.