Emma (The Curse of Fatal Death)

Emma was a companion of the Ninth Doctor, as well as his fiancée. After the Doctor regenerated several times in the space of a few minutes, she maintained her engagement with the Tenth, Eleventh and Twelfth, 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
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)

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)

Behind the scenes

 * The idea of the Doctor falling in love with a companion, and vice versa, would be revisited several times when the TV series returned in 2005.
 * Teleplay writer Steven Moffat would later write for more characters with whom the Doctor fell in love, including Madame de Pompadour (TV: The Girl in the Fireplace) and River Song. (TV: Day of the Moon, Let's Kill Hitler)
 * 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.