Who is the Doctor's granddaughter?

The Doctor's granddaughter is Susan Foreman.

Susan first appeared in An Unearthly Child, the opening Doctor Who story broadcast in November 1963. She travels with her grandfather in the TARDIS and is one of the programme's original companions, alongside her teachers Ian Chesterton and Barbara Wright. Her unusual knowledge and apparent familiarity with the TARDIS help draw Ian and Barbara into the Doctor's adventures.

Susan is portrayed by Carole Ann Ford and remains with the First Doctor for the first season and the opening stories of the second. Although the programme identifies her as the Doctor's granddaughter, it never names her parent or explains the wider family relationship in detail. That deliberate mystery helped reinforce how little viewers initially knew about the Doctor's origins.

Susan is sometimes confused with later companions such as Rose Tyler, Amy Pond, or Clara Oswald. They are important travelling companions, but none is the Doctor's granddaughter. Susan eventually leaves the Doctor in The Dalek Invasion of Earth, when she chooses to remain in 22nd-century London with David Campbell.