9. Tish (Series 3 Recurring)
Given that the interactions between Tish and Martha were really the only redeeming aspects of The Lazarus Experiment, I think the writers really missed a trick in not letting her travel in the TARDIS for a few episodes. She's proven that she's brave and intelligent and seems far more naturally sympathetic to the Doctor than her shrewish mother. And if there had been an added sister dynamic, maybe less of Martha's personal storyline would have revolved around making moon eyes at the Doctor. It would have been cool to have a companion and her sister travelling around for a few episodes...and it might have even raised the stakes for the Season 3 finale. Look, all I'm saying is that if the Doctor was willing to let Jackie into the TARDIS, Tish should have been an easy decision.