First Appearance (in comics): I'm not really sure.
Appearance: The Mystery of Fu Manchu, Marvel Special Edition #15, and various Marvel Comics.
Years Active: 1913-present.
Sir Denis Nayland Smith was, in Sax Rohmer's "Fu Manchu" books, the stalwart, square-jawed British opponent of the Insidious Doctor. He holds the same role in the Marvel comics in which the Insidious Doctor Many Men Smoke But appears, although the Doctor's son, Shang-Chi (the Master of Kung Fu), is the center of the tales.
If we take the original dates of the novels to still be accurate in Marvel chronology, then Smith was in at least his mid-twenties in 1913, which would make him extremely old during the modern era, although undoubtedly there's a good, in-continuity reason why he looks so much younger in the Shang-Chi stories than he is. Smith began with Scotland Yard, moved to the CID, and then finally went freelance, forming his own team, Freelance Restorations. Smith had the usual secret agent skills.
Note: Michael Hoskin notes that "some Fu Manchu fans have guessed that his vitality in contemporary novels & comics is due to having taken the Elixier Vitae at some point" and that Black Jack Tarr is the new director of Freelance Restorations, Smith being said to have retired.