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Martha is the most professional companion (which sounds like a hooker, but I assure you is not). Instead of just being enthusiastic, or charming, or even just brave like many of the other companions, she was all those things with a business-like layer of efficiency on top. Often working to save the day/alien race/world in the Doctor's stead, she was the most skilled companion at working with the governments, armies and liaisons of the threatened planets (or times). Some highlights? Helping to defeat the Master (the Doctor's closest frenemy), aiding the Torchwood team, and even tangling with the Daleks. The next companion after the Doctors's pseudo-girlfriend Rose, Martha and the Doctor never fully gelled as co time-and-space travelers. He was still sore over losing Rose and Martha's inevitable crush on the compelling alien left her feeling second-fiddle to the memory of Rose. Smart, bright and sharp, Martha was never a mere observer to the madness that is the Doctor, she was an essential component to the story; succeeding where he couldn't and looking beyond her own needs and desires to what was most important. Martha ultimately chose to return to the world of normals to be there for her family, but she was propelled back into the Doctors sphere in The End of Time Part II, where we found her happily married to another former companion Mickey Smith (yay) and still saving the day like a champ. Martha's time with the Doctor was important, but I couldn't help wishing it had been a bit more joyous. Martha drew the short straw by joining him at a time when he was the most maudlin- but it did work out in the end, and imagine the stories Mickey and Martha will have to tell the grandkids!
Note: Details remembered with an assist from IMDB character biography page.