He first appeared welcoming Matthew, a Shoreditch policeman who was killed by a Skovox Blitzer, to the Nethersphere as Missy was busy at the time. He then proceeded to ask Matthew if he had "any questions." (TV: The Caretaker)
He also welcomed Danny Pink to the Nethersphere. He offered the man a coffee and asked if he had arranged to be cremated. Seb told Danny that he had died, and that he could still feel his body, even though his consciousness 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 Danny. He watched as the two talked, and Danny broke down after Clara ended the call. Seb then gave him the chance to delete his emotions, leaving him alone to make the decision. (TV: Dark Water)
He was in the Nethersphere with Danny and the boy he killed stating that now the dead were going back home as all the lights in the Nethersphere turned off. After that, he appeared in the Nethersphere with Missy watching the Twelfth Doctor and the TARDIS free falling from a plane. He showed clear signs of being impressed when the Doctor managed to open the TARDIS, and complimented the Doctor. Annoyed by Seb, Missy casually killed him with her weapon. (TV: Death in Heaven)
Seb presented himself as a charming, polite and affable individual. He made sure to be charitable when greeting new deceased peoples, asking them questions - usually asking them if they were being cremated or if they were curious about the Nethersphere - while trying to not be too pushy and sometimes offered them a coffee, like he did with Danny.
He was also known to be sarcastic and enjoyed making sometimes inappropriate jokes and quips. Some examples are like when he's purposefully vague about where confused deceased peoples are, remarking that "he can see his house from here" when Danny was looking out the window before apologizing as he felt that it "wasn't helping". He also asks, ambiguously genuinely, if Danny would like a bag to breathe into when he begins hyperventilating in shock.
Despite this, he has shown the villainous side of his personality on select occasions, such as smiling when Matthew shows confusion about where he is and how he "escaped" and in a way, manipulating Danny and probably plenty of other people into willingly deleting their emotions to help their conversion into Cybermen.