Rose Tyler ends her story with a version of the Doctor, with a mission to heal his battle scars. But a Companion’s job is greater than healing her Doctor and keeping him grounded, or even saving his life on occasion. In fact, the Companion must have perfect faith in the Doctor, one that transcends all obstacles. This belief gives him the responsibility to stay ethical in all situations. Donna Noble insists he acknowledge his daughter Jenny, and she demands that he save a single family from the fires of Pompeii. The Doctor asks Martha to watch over him when he turns human but her greatest task comes during the year that never was. She travels the world, inspiring everyone, person by person, to believe wholly in the Doctor, as she does. As she insists he can save the world, the belief makes it happen.
Even traveling with the Doctor takes a tremendous amount of faith, as a stranded passenger would never make it home. In fact, when Adam Mitchell (with a zipper in his head) breaks the rules, the Doctor honorably returns him to his old life. Even returning Sarah Jane to Scotland instead of her old home is a minor inconvenience.
Katarina, who worships the First Doctor as the god Zeus, is an unfortunate example, who was quickly written out. But the other Companions show faith in not just the Doctor but all of humanity: Barbara Wright believes the Aztecs can change (“The Aztecs”). As Amy Pond sits, eyes closed, resisting the weeping angels, she exemplifies the perfect faith and belief required of a Companion. She shows faith in others as well, trusting the Star Whale, and invoking the humanity in a World War II android.
Faith leads to faith and trust to trust, inspiring the Doctor in his quest to exemplify the best of humanity, with the best of humanity by his side.