Martha Jones was played by Freema Agyeman who joined the Doctor in 2007, she also appeared in Torchwood.  Martha begins as a medical student who becomes The Doctor's time travelling companion after an incident at the hospital where she works. She chooses to leave the Doctor after realising that she is in love with him but that he does not return her feelings. She returns to Earth and finishes her degree, she then joins UNIT.  Martha's story arc is incredible and she is a brilliant role model.  She was with the Doctor from 2007 to 2010