Emma (The Curse of Fatal Death)

Emma was a companion of the the Doctor's. 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 subseqent series of rapid-fire regenerations, ultiminating into a final, female regeneration, Emma broke off the engagement, saying the Doctor was, in every sense of the word, no longer the man she fell in love with. (CP: The Curse of the Fatal Death)

Behind the scenes

 * The storyline of Curse of Fatal Death is not considered canonical.
 * 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 create more characters with whom the Doctor falls in love, including Madame de Pompadour (DW: The Girl in the Fireplace) and River Song (DW: Silence in the Library/Forest of the Dead).