Frazer Hines
Hines' "Doctor Who" debut came in 1966, when he was cast to play the part of Jamie McCrimmon, the companion of the Second Doctor (played by Patrick Troughton). Originally intended as a one-off guest character, Jamie joined the regular cast and appeared in the series from 1966 to 1969. Hines reprised the role in a cameo in the 20th anniversary serial "The Five Doctors" (1983) and as a guest star in "The Two Doctors" (1985). Hines would appear in 117 episodes of "Doctor Who" in all—more than any other "companion" actor in the history of the series. The only actors appearing in more episodes are those who played the first four Doctors. Many of the episodes featuring Jamie no longer exist in the BBC's collection.
Jodie Whittaker
On 16 July 2017, the BBC announced that Whittaker would become the thirteenth and first female incarnation of The Doctor in the British TV series "Doctor Who". She formally assumed the role from Peter Capaldi in the 2017 Christmas special episode "Twice Upon a Time" when Capaldi's Twelfth Doctor regenerated into Whittaker's Thirteenth Doctor. Whittaker appears in her first full series as the Doctor in the eleventh series, which premiered in October 2018. She is set to continue in the role in the twelfth series in 2020.