Sabbath was an enemy of the Eighth Doctor.
Biography
Sabbath was born in 1740. He studied at Cambridge and worked for the Secret Service before defecting in 1780.
Sabbath encountered "Leviathan" under the River Thames as part of his indoctrination ceremony into the Service. He set up his own power base independent of the Service. After leaving the Service, Sabbath sent a message via a number of corpses of the Service's "ratcatchers" saying "leave me alone, and I'll leave you alone".
The Eighth Doctor chose Sabbath for his best man at his wedding to Scarlette. Sabbath later removed the Doctor's second heart, allegedly to cure the Doctor of an illness caused by the heart's attempt to connect the Doctor to the non-existent Gallifrey, but also allowing Sabbath to travel through time in the Jonah, endowing him with the Doctor's ability to penetrate 'deep time'- exploring the universe beyond his home era and planet- that would have been impossible in his more primitive ship otherwise. (PROSE:The Adventuress of Henrietta Street)
Sabbath had agents which were to him the same as companions were to the Doctor. Among them were: Juliette Vierge (PROSE:The Adventuress of Henrietta Street); Sasha and Jueves; (PROSE: History 101) Jaxa and Roja, former Time Agents; (PROSE: Trading Futures) and The Angel Maker (aka Elizabeth Kelly). (PROSE: Camera Obscura)
Sabbath arranged for the Doctor and the Clock People to meet on a Plutocratic Empire colony planet. (PROSE: Anachrophobia)
While near death, Sabbath met Death, learning that the Doctor's second heart created a link between the Doctor and Sabbath. Wanting to end this link, Sabbath tore out the heart, allowing the Doctor to grow a new one. (PROSE: Camera Obscura)
Having been manipulated into taking actions that would allegedly 'collapse' reality into a single manageable timeline, Sabbath later assisted Fitz Kreiner, Anji Kapoor and the Doctor when multiple realities began to converge and mix. (PROSE: The Last Resort)
Sabbath was finally killed after turning his back on his employers and helping the Doctor; realising that his employers gained power from predicting future events, and with their plans now dependent on Sabbath killing either the Doctor or the head of the Council, Sabbath instead shot himself, something that the Council 'knew' he wouldn't do. (PROSE: Sometime Never...)
Personality
Sabbath was generally opposed to chaos and believed that unauthorised time travel created parallel universes which threatened reality. Sabbath was confident in his mental abilities. Aided by the Doctor's second heart, he survived a journey into Death's realms. (PROSE: Camera Obscura)
He was incredibly stubborn and disliked beings who lorded their powers over others, including the Doctor. However, this arrogance also meant that he found it hard to admit when he was wrong, constantly convinced that he understood time better than the Doctor even after the Doctor provided evidence that time travel did not work the way Sabbath beieved it did (PROSE: Time Zero).
Sabbath used many pseudonyms, often variations upon his chosen name of Sabbath, such as Holiday or Mistletoe. (PROSE: Anachrophobia, Time Zero) Although he openly scorned his agents, he was fiercely loyal to them. (PROSE: The Adventuress of Henrietta Street)
Although intended to oppose the Doctor, Sabbath's actions, such as his focus on manipulating others and long-term plans, echoed the Seventh Doctor more than the Eighth, although his masters' manipulations meant that he made various mistakes that the Seventh Doctor would not have made.
Appearance
Sabbath was bulky and muscular. His body was getting fat, having concluded that he was better served as a behind-the-scenes manipulator rather than taking action himself. He kept his head almost shaved. This was not a political statement or an attempt to intimidate people, but to save time and energy in grooming. (PROSE: The Adventuress of Henrietta Street) Many of Sabbath's clothing choices echoed the Doctor's, for example, he wore a linen suit after adopting the role of Time's Champion. (PROSE: Camera Obscura)