Martha Jones

 Dr Martha Jones, later Martha Smith-Jones, was a British physician. She met the Tenth Doctor while in residency at Royal Hope Hospital in London when the hospital was transported to the moon by the Judoon, after which she began travelling with the Time Lord. (TV: Smith and Jones)

Unlike other companions, she not only fell in love with the Doctor but also declared her feelings. She decided to stop travelling with him as he failed to return that affection, but continued to have a number of adventures both with and without him, thanks to her work with UNIT and Torchwood (TV: Last of the Time Lords, Reset, etc.).