BRIDGET (BIDDY): A with-it girl of 15, reaching the end of her Secondary School career, eager for life, lower-than-middle class. Avoid dialect, use neutral accent laced with latest teenage slang.
That is the first description of the character that was to become Susan, as written by Donald Wilson, C. E. Webber, and Sydney Newman.
Carole, meanwhile, remembers the character as being sold to her as being much tougher than she eventually became, more like the Cathy Gale character that was appearing in The Avengers at the time and having some of the alien qualities of the character of Andromeda in the BBC series A for Andromeda and The Andromeda Breakthrough. However, the only alien quality she exhibits is her telepathic abilities in "The Sensorites."
The notion that she was the Doctor's granddaughter came from Anthony Coburn, the writer of the first story, who thought it improper that the Doctor was traveling around with an attractive young woman not related to him. So much for that.
Back to the TARDIS.