Sydney Smith

Sydney Smith was John Smith's father in the fictional backstory John thought was his own.

Sydney was a watchmaker. He and his wife Verity raised John on Broadmarsh Street in Radford Parade in the district of Nottingham, where Sydney was from. (TV: Human Nature) After he and his wife died the family maid, Martha Jones, continued to serve John. (TV: The Family of Blood)

John Smith was in fact the Tenth Doctor, and these memories were a constructed part of his identity after the Chameleon Arch made him human. (TV: Human Nature, The Family of Blood)

Behind the scenes
John's parents' names Verity and Sydney were a tribute and reference to Verity Lambert and Sydney Newman, both essential to the creation of Doctor Who.