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".
River Song (Doctor Who)
River Song is a fictional character created by Steven Moffat and played by Alex Kingston in the British science-fiction series "Doctor Who". River Song was introduced to the series as an experienced future companion of series protagonist the Doctor, an alien Time Lord who travels through time in his TARDIS. Because River Song is a time traveller herself, her adventures with the Doctor occur out of synchronisation; their first meeting (from the audience's perspective) is his first and apparently her last. While other actresses briefly portray younger versions of the character, Kingston plays her in 15 episodes, as River becomes a companion, romantic interest and eventual wife of the Doctor in his eleventh incarnation portrayed by Matt Smith.