Emma (The Curse of Fatal Death)

Emma was a companion of an individual calling himself the Ninth Doctor, as well as all his known successors.

Biography
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 final, 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. (NOTDWU: 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.