Susan Foreman is the main deuteragonist in Season 1 and a major protagonist in Season 2 of Doctor Who.
She was portrayed by Carole Ann Ford, who later reprised the role in the Big Finish audio plays. She was also voiced by Claudia Grant for Big Finish and by Jane Asher on the radio.
History
Susan was the Doctor's granddaughter, who had been travelled with him for some time before they arrived in England in 1963. She attended the local school, Coal Hill, where the gaps in her knowledge, coupled with expertise in other subjects, attracted the attention of teachers Ian Chesterton and Barbara Wright. When they followed her home to the TARDIS, the Doctor decided he and Susan needed to leave and, when Susan threatened to stay behind without him, he set the craft in motion with Ian and Barbara still on board, condemning the four of them to aimless wandering.
Not long after, Susan encountered the Daleks for the first time on a visit to Skaro. She was responsible for saving her friends from radiation poisoning by braving a journey into the forest alone to retrieve anti-radiation drugs. During a lengthy journey across Cathay, she befriended Ping-Cho, a girl of her age being taken to an arranged marriage. During a visit to the time of the Aztecs, Susan's outspoken nature caused her to run afoul of the local culture, as she refused to marry the Perfect Victim, an intended sacrificial victim whose wishes could not be denied. She was nearly mutilated as punishment before the Doctor found a way back to the TARDIS.
On the planet of the Sensorites, Susan found the local conditions enhanced her latent telepathy. She was the first to realise the Sensorites were benevolent and was able to use her abilities to guide the Doctor and Ian through the catacombs in order to uncover the culprits who had been poisoning the aliens.
During her second encounter with the Daleks, who were occupying 22nd century Earth, Susan fell in love with the freedom fighter David Campbell and was confused by his desire to settle down when the Daleks were defeated. David asked her to stay and marry him but Susan was reluctant to leave the Doctor. Sensing she would be happy there, the Doctor shut her out of the TARDIS, saying goodbye to her via the ship's scanner.
Many years later, when Susan was a grown woman, she was abducted by the corrupt Time Lord President Borusa and transported to Gallifrey as part of his scheme to gain immortality. She was reunited with the First Doctor, also encountering three of his future incarnations. She was presumably returned to Earth after the incident.
There are many contradictory accounts of Susan's future in various spin-offs. The text story "A Time and a Place" indicated she settled happily on Earth and had a daughter, Barbara. The novel "Legacy of the Daleks" claims that she aged slower than humans and was unable to have children with David, leaving their marriage tense. After David was killed by the Master, she defeated the evil Time Lord and left in his TARDIS. The Big Finish audio plays revealed she and David had a son, Alex, but David later died. She had several adventures with the Eighth Doctor, encountering the Daleks again.