For the duration of the Tenth Doctor adventure Planet of the Dead, Lady Christina de Souza, played by Michelle Ryan, filled the position of companion (or assistant, if that sounds better). At the end of that story, the Doctor decides he's better off travelling alone and leaves a disappointed Christina behind in London. Faced with imminent arrest, Lady Christina flees from the stalwart officers of the Metropolitan police force and, with the Doctor's help, steals the anti-grav-equipped bus, handily making her escape. Michelle Ryan imbued the part with plenty of energy and charm - she was fun, and isn't that what Doctor Who needs? It's likely that another appearance would be equally as enjoyable for the audience. In fact, Lady Christina's return is such a popular idea that in an IDW comic (The Eye of Ashaya), she did return to have a brief adventure with the Eleventh Doctor, this time turning down the Doctor's offer to take her on further adventures and making it into space under her own steam. What goes around comes around, Doctor.