Seb is an Artificial Intelligence Interface created by Missy from Doctor Who to help her with greeting and assimilating new deceased people in the Nethersphere.
He is played by Chris Addison.
Seb was first introduced at the end of Series' 8 episode "The Caretaker", meeting Matthew; a police officer who had been killed earlier on.
When Matthew starts to become confused about where he is and how he "escaped", Seb informs him he's in the Afterlife, or the "Promised Land", or as he enjoys calling it; the "Nethersphere". When Matthew proceeds to look out the window, Seb joins him and their attention is drawn to Missy, who proceeds to give them a cold stare and walk away. Seb apologizes for her being busy and asks Matthew if he has any questions.
Seb then reappeared in the two-part finale of Series 8 to greet Danny Pink, who had been struck by a car and killed. He helps Danny get adjusted to his new "life" and sets up a meeting between him and a boy he had accidentally killed during his time as a soldier.When the boy runs away in fear, Seb tells Danny to not follow him. He also explains that Danny's consciousness is still linked to his body, which would explain the screams of a man that had "left his body to science" and why Danny felt cold.
After Danny gets a call from Clara Oswald, his girlfriend, Seb stands in the background and watches as Clara questions Danny to see if he's real. When Clara angrily hangs up and Danny subsequently breaks down into tears, Seb offers him the choice to delete his emotions, leaving him alone to make a decision.
When the boy Danny killed appears again, thus leaving Danny to decide to not delete his emotions, Seb comes back and the three of them watch as the lights in the Nethersphere go out. Seb informs them that this means they're going back to their bodies, and that there's been an "upgrade".
Later on, Seb and Missy watch as the Twelfth Doctor fall from a plane after it blows up and Seb becomes visibly impressed when the Doctor is able to call his TARDIS to him to save his life. As he grows excited, Missy becomes annoyed and vaporizes him.