(Carole Ann Ford)
Susan was the first companion for the First Doctor. She was the Doctor's granddaughter, and she started out all weird and interesting, but by her fourth or fifth story had become the first clichéd "Doctor Who girl", all screams and twisted ankles.
Here's Susan being all kick-ass and awesome holding a flaming skull to scare off some cavemen, in An Unearthly Child, the very first Doctor Who serial.
And here she is still awesome in the third serial, The Edge of Destruction, as she's driven insane by the TARDIS and tries to kill her science teacher with a pair of scissors.
And here she is in her final serial, The Dalek Invasion of Earth, with a twisted ankle. Not shown: endless screaming.
It was in this story that the Doctor left her behind, locking her out of the TARDIS on a war-ravaged future Earth to live out the rest of her life with a boy she just met. He promised to one day come back, and then never mentioned her again.
Susan was time-scooped home to Gallifrey in the 20th anniversary special The Five Doctors, where she met several future incarnations of her grandfather, none of whom seemed particularly excited to see her. She hasn't been mentioned specifically in the new series, but the implication is that, like the rest of the Doctor's family, she's dead. Presumably a twisted ankle was involved.