As for the companions, each made her mark. The first real romance in Doctor Who history, between Billie Piper’s Rose and the Doctor, gave the series a whole new audience. As Martha Jones in season 3, Freema Agyeman became the first black companion, and as a medical student training to be a doctor, she was higher in the middle class than Rose. Catherine Tate as Donna brought yet another dimension, which is hard to describe: she was part of some great comic moments with Tennant, who also does comedy well ( their most comic episode, about Agatha Christie, is one of my favorites in the entire series), but she also brought a particular kind of human vulnerability within her sassiness.
Still, it’s worth noting that both Rose and Donna saved the Doctor’s bacon and the universe by becoming almost godlike, and both paid a heavy price, while Martha was humanly heroic, and survived her TARDIS time—though (as Donna noticed) she was transformed from a healer into a warrior. Just part of the doubleness of the Doctor and his disciples, and one of the paradoxes of the Doctor’s journey.