Emma (The Curse of Fatal Death)

Emma was a companion and the fiancée of the Ninth Doctor. After the Doctor regenerated several times in the space of a few minutes, she maintained her engagement with the Tenth, Eleventh and Twelfth Doctors, but reluctantly broke up with her beloved's thirteenth incarnation because that body had come out female-presenting and Emma had no sexual or romantic interest in women.

Biography
The Ninth Doctor once wrote in a letter to a newspaper that his "glamorous assistant" Emma would appear in the televised "Comic Relief spectacular". (PROSE: Who's After Your Cash)

Unlike past companions, the Doctor fell in love with Emma and proposed marriage to her, indicating his intent to finally settle down. However, after a subsequent series of rapid-fire regenerations culminating in a female incarnation, Emma broke off the engagement, saying the Doctor was, in every sense of the phrase, "no longer the man [she] fell in love with". (TV: The Curse of Fatal Death)

Other references
When the Eighth Doctor looked into the Tomorrow Window, he saw several possible ninth incarnations, one of whom was "a listless-looking man [who] sat on a sofa beside a girl in a red dress in an unconvincing medieval dungeon". (PROSE: The Tomorrow Windows)

Behind the scenes

 * According to the behind-the-scenes documentary, Sawalha called Louise Jameson for advice on how to convey the intelligence of the Doctor Who companion.
 * Also in the behind-the-scenes documentary, Sawalha commented that she'd previously been offered a part in the Doctor Who series proper, but "didn't fancy it at the time." She appreciated the chance to star in "the spoof version of it" instead.