Sarah-Jane Smith is a fictional female Human in the Doctor Who series. The character was portrayed on screen by Elizabeth Sladen. Her allegiance / loyalty is to Third Doctor. Sarah appeared during the Tom Baker years along with K9. When Sarah left the Doctor, he gave her the third K9 as a present.
Although retired from the series, she popped up now and again in special episodes. These included The Five Doctors and "Dimensions in space". There was an attempt of a spin-off series featuring K9 and Sarah but it didn't get very far past the pilot production. Sarah and K9 are due to return in the first series of the Doctor staring David Tennant.