Sabetha

Sabetha was a young woman sent by Arbitan, her father, to find the keys to the Conscience of Marinus, but instead became enslaved in the city of Morphoton. After being freed, she accompanied the Doctor and his companions in their search for the keys. (DW: The Keys of Marinus)

Behind the scenes

 * Although rarely recognized as such, Sabetha is actually an early example of a "one-off" companion who accompanies the Doctor for the duration of one story only. Sara Kingdom, who is often considered a full companion, is another example.