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.
Romana (Doctor Who)
As a Time Lord, Romana is able to regenerate, having had two on-screen incarnations with somewhat different personalities (dubbed Romana I and Romana II by fans). Romana I was played by Mary Tamm from 1978 to 1979. When Tamm chose not to sign on for a second season, the part was recast. Romana II was played by Lalla Ward from 1979 to 1981. A third incarnation of Romana has been depicted in some of the spin-off novels, and a fourth (performed by Juliet Landau) has been featured in several audio dramas released by Big Finish Productions in 2013 and 2014, and appeared again in early 2015.