She was a companion to the fourth Doctor (Tom Baker), and is herself a Time Lord or perhaps more accurately, a Time Lady. She had two incarnations, one played by Mary Tamm (who chose not to sign on for a second season, despite optimistic prodding on the part of the show’s producers)and the second played by Lalla Ward (who later became Tom Baker's wife).
Romana first appeared in The Ribos Operation. Her final appearance was in Warrior's Gate. She also appeared briefly in The Five Doctors through the reuse of footage from the uncompleted story Shada and has made appearances in several subsequent novels and Big Finish audio Doctor Who adventures. Virgin Books's New Adventures and BBC Books's Eighth Doctor Adventures as well as Big Finish's audio productions have established her as Lady President of Gallifrey).
She was originally assigned to assist the Doctor during the quest for the Key to Time (a story arc which spanned a whole season). The quest complete, she regenerated, emerging with a more light-hearted personality.
This arbitrary regeneration was the idea of Douglas Adams, script editor of the series at the time. Adams was criticised by some fans for introducing too much of the sort of humorous content that served him well in The Hitchhiker's Guide to the Galaxy into the season on which he served as the script editor. However, other fans consider some of Adams' scripts to be among the series' high points.
In particular, the regeneration scene (in Destiny of the Daleks, a script credited to Terry Nation, but with several additions and alterations by Adams, including this scene) has long been a source of controversy. The cause was its light-hearted approach to the subject: Romana instantly changing forms several times, rather like someone changing clothes, before taking the form of Lalla Ward (who had played Princess Astra in the final story of the previous season). The Doctor Who role-playing game by FASA offered an explanation that Romana has the ability to control her regenerations which enabled her to adjust her appearance in the process to suit her tastes.