Emma (The Curse of Fatal Death)

Emma was a companion of the Doctor in a possible future.

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. (DW: 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 (DW: The Girl in the Fireplace) and, possibly, River Song. (DW: Silence in the Library / Forest of the Dead)
 * According to the behind-the-scenes documentary, Sawalha called Louise Jameson for advice on how to convey the intelligence of the Doctor Who companion.