A genial man in a bowler hat reading a newspaper beside a free-standing door on Dannus V, who proves to be the Guardian of Forever and judges whether Philippa Georgiou deserves to be saved.
Carl appears in the two parts of Terra Firma, played by Paul Guilfoyle. Georgiou, a Terran displaced in both time and universe, is dying as her body comes apart, and Discovery's data points to a snowbound world where a cure may exist.
There she and Burnham meet Carl, whose newspaper carries tomorrow's report of Georgiou's death. He invites her through the door, and she relives a crucial stretch of her reign in the Terran Empire, this time showing mercy.
On her return he reveals himself as the Guardian, in hiding since the Temporal Wars because people tried to use him as a weapon. Satisfied that she has changed, he sends her to a time when the two universes lay closer together and she can live.