Sydney Smith

Sydney was the husband of Verity and the father of John Smith in the fictional backstory John thought was his own.

Sydney was a watchmaker. He and his wife 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 the Doctor 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 making of Doctor Who.