It is strongly implied that he is the younger version of fictional detective Nero Wolfe in the mystery series by Rex Stout.
Lupa also shares several of Wolfe's attributes, including a tendency to be overweight, a love for fine cooking, orchids, and beer, distaste for the female sex, and a preference for the color yellow.
Author William S. Baring-Gould previously postulated that Nero Wolfe was the son of Sherlock Holmes and Irene Adler in his books Sherlock Holmes of Baker Street (1962) and Nero Wolfe of West Thirty-Fifth Street, the life and times of America's largest private detective.
In Son of Holmes, Lupa investigates the murder of an intelligence agent and acts of sabotage in a small French town during World War I.
In Rasputin's Revenge he travels to Russia at the request of Tsarina Alexandra and investigates the murder of a relative of Tsar Nicholas II.