Doctor Who
Thirteen actors have headlined the series as the Doctor. The transition from one actor to another is written into the plot of the show with the concept of regeneration into a new incarnation, a plot device in which a Time Lord "transforms" into a new body when the current one is too badly harmed to heal normally. Each actor's portrayal is unique, but all represent stages in the life of the same character. Together, they form a single lifetime with a single narrative. The time-travelling feature of the plot means that different incarnations of the Doctor occasionally meet. The Doctor is currently portrayed by Jodie Whittaker, who took on the role after Peter Capaldi's exit in the 2017 Christmas special "Twice Upon a Time".
Jack Harkness
Captain Jack Harkness is a fictional character played by John Barrowman in "Doctor Who" and its spin-off series, "Torchwood". The character first appears in the 2005 "Doctor Who" episode "The Empty Child" and subsequently features in the remaining episodes of the 2005 series as a companion to the series' protagonist, the Doctor. Subsequent to this, Jack became the central character in the adult-themed "Torchwood", which aired from 2006 to 2011. Barrowman also reprised the role for guest appearances in "Doctor Who" in its 2007 series and 2008 series, as well as the two-part special "The End of Time".