Seb was a man who worked for Missy in what he liked to call the Nethersphere. He welcomed Matthew, a Shoreditch policeman who was killed by a Skovox Blitzer, as Missy was busy at that time. (TV: The Caretaker)
He also welcomed Danny Pink to the Nethersphere. He offered the man coffee and asked if he had been cremated. Seb told Danny how he had died and that he could still feel his body, but his conscience was no longer a part of it. He also arranged a meeting between Danny and a boy he had once killed while on duty. When the boy fled during the meeting, Seb told Danny to allow him to leave.
Seb allowed Danny to talk to Clara Oswald, after the latter had, quite "rarely" in his opinion, managed to call them. He watched as the two discussed and Danny break down after Clara ended the call. Seb then gave him the chance to delete his connection to his body, leaving him alone to make the decision. (TV: Dark Water)
Seb is actually the Master. Rassilon killed him after the events of 'The end of time' He was uploaded to the Matrix, which he was able to take control of after Galifrey was hidden in a pocket universe. He was able to talk to Missy, who is actually the Doctors wife, and persuade her into striking an alliance with the Cybermen. Together Seb, Missy and the Cybermen took control of the earth by sending all of the dead to the Matrix 'heaven' there they would be upgraded into Cybermen. Seb possessed Missy and made her believe that she was the Master, Missy was then able to take the Doctor to the Matrix -which Seb couldn't leave due to having died and being uploaded- When the Doctor met Seb He was informed of Sebs plan to force the Doctor into making the Time Lords return so that they could be defeated by Sebs army of Cybermen.