David Smart (in-universe)

David Smart was a ringmaster in a circus, where his sister, Gabriella, performed with dogs. (TV: Billy Smart's Children's Circus)

Biography
While waiting for Gabriella to do her act, the Third Doctor showed up in the Whomobile with her and one of her dogs, having been helping her look for them when she lost the dogs. Gabriella then went off again with the Doctor to find the rest of her dogs. (TV: Billy Smart's Children's Circus)

Behind the scenes
David Smart portrayed himself in Billy Smart's Children's Circus.