- Frank Buck
- Frank Hardy
- Sasha Siemel
- 'Tiger' Van Dorn
- Reginald Denny
- Ralph Marshall [Chs.1-3]
- Esther Ralston
- Valerie Shields [Chs. 1, 3, 6, 7, 15]
- Charlotte Henry
- Dorothy Elliott
- William Bakewell
- Tom Banning
- Clarence Muse
- Lightning - Street Singer
- Willie Fung
- Chiang - Houseboy
- LeRoy Mason
- Jim Murphy
- Matthew Betz
- Inspector Starrett
- Richard Tucker
- Robert Banning
- Duncan Renaldo
- Armand Roget
- Fred Kohler Jr.
- Det. John Charles - alias Quinn [Chs. 5-6]
- Sherwood Bailey
- Paul Marshall [Chs.2-3, 6, 8-10]
- 'Snub' Pollard
- Campbell - Hardy's Aide [Chs. 1, 2, 4, 7, 8]
- George Rosener
- The Professor
- Gertrude Sutton
- Mrs. Maitland - Housekeeper
- John St. Polis
- Chandler Elliott [Ch. 1]
- John Davidson
- Dr. Coleman
- Milburn Morante
- Singapore Joe [Chs. 2, 7, 9, 15]
- Dirk Thane
- Joe Nolan [Chs. 1, 5, 6]
- Robert Warwick
- Chief Insp. Angus MacLeod
- Dahlam Ali
- Hardy's Assistant
- Jim Corey
- Henchman
- Paul Ellis
- Greenway
- Henry Hale
- Jackson
- Harry Harvey
- 1st Roget Clerk
- Reed Howes
- Sailor
- Jack Ingram
- Undercover Detective at Dive
- Tom London
- Detective Interrogating Pete
- Merrill McCormick
- Peters
- Blackie Whiteford
- Henchman
- Roger Williams
- Henchman
