Mystery Men & Women:

The I's

  Icarion: 1940, Colossus Comics #1 (Sun Publishers). "Child of the Sun". Featured in an ad for the second issue which never came about, Icarion is pictured as apparently a winged maskless hero with curly blond hair, t-shirt, trunks and winged shoes ala the Flash hoisting a soldier over his head. His name is obviously a play on Icarus.
 

Igor the Archer: 1947, International Comics #1 (EC). Igor the Archer is a Robin-Hood type hero only he operates in Russia of the Middle Ages. His father is a nobleman who is killed by the tyrant czar on the throne while he is branded an outlaw. Igor decides to oppose tyranny wherever he finds it, using his skill with the bow. The character is eventually transplanted to the present day (his descendent) and this Igor is an international policeman, part of the International Crime Patrol reporting to the U.N.

  Illuso: 1944 or 1945, Everybody's Comics/Rocket Kelly #1 (Fox). Information is scarce concerning this hero. He is a magician type, schooled in the arts of magic in Tibet. Note: There were two Rocket Kelly comics by Fox. One, a one-shot in 1944 and the second a small run in 1945. The GCD lists Illuso only in the first issue of the second but the index information is sparse on details of both. Illuso is also listed as being in Everybody's Comics which also came out in 1944, but the information of that comic and its features are likewise bare bones.
  Impossible Man: 1944, Red Band Comics (Enwil/Rural). Hugh Mann (human... ha ha) is puny, even by Earthling standards. He's also a bit of a failure in life, no job, no girlfriend. He's generally smart and clever though, so to prove his worth, he builds a spaceship to take him to Mars. That screws up too as the controls break and he meanders about space until he crashes on the lost planet of Brutus (which his charts show). On this planet, everyone is super: super strenght, flight, super invulnerable. So they call themselves that: Super Cop, Super Judge, etc and he's just a super-weakling. To the point, they consider him a freak and cover him with cotton padding to protect him. However, when they are threatened by Super Phony who has super hypnotism, Hugh takes on the task of stopping him. Which he does with the aid of a mirror, reflecting back the hypnotism. Since he succeeded at an impossible task, Hugh is now the Impossible Man.
 

Inner Circle: (1939) Amazing Mystery Funnies v2 #7 (Centaur) An international group of men gather to straighten out a lot of wrongs in the world. They were assisted by a support group called the "Outer Circle". Guess ole Adolph somehow slipped by. Major Ramsey: A member of U.S. Army. Colonel Ewan. Lieutenant Michel Dupre: A French soldier. Rolfe Gunderson: Norwegian scientist and explorer. Ian Ahern: From Northern Ireland and the youngest. John Bain: A Canadian engineer.

  International Crime Patrol: 1947, International Comics #1/ 1948 International Crime Patrol #6 (EC). The patrol answers to the UN and are lead by Van Manhattan. Other members include the Chessmen, Igor the Archer and Madelon, all who headline their own strips. NOTE: The group operated mainly in solo stories, connected by sending radio messages between them.
  Invisible Avenger: 1940, Superworld 1 & 2 (Komos Publications). Buzz Allen is a young ham radio operator is orphaned when his father is killed by racketeers. He accidentally discovers the power of invisibility when installing a "super-electron tube" to increase its power, it instead turns the radio invisible. He turns his discovery into an invisibility belt and goes after the racketeers that killed his father with the aid of buddy Will Lawrence, set out to do the righting-wrongs thing, starting with the racketeers. From there, they continue to have adventures and fight crime. In issue 3, they prevent an invasion of America by Phatso, the leader of the lost city of Atlantis (being in the vicinity of Alaska, not to difficult too see why Atlantis has been so hard to find).
  Invisible Scarlet O'Neil: 1939 Comic strip then Famous Funnies 81-167, a seven year run from "41 to "48. Poking around her father's lab, Scarlet touches a "weird looking ray" which grants her the power of invisibility whenever she touches a nerve in her left wrist. The ray itself never works again and her father dies trying to fix it. Her boyfriend is Sandy. Created by Russell Stamm, Scarlet is the first hero with the sole power of invisibility and one of the first superheroines.
Invisible Terror: 1942, C-M-O Comics (Centaur). Drifter Tom Wade accepts the position of assistant to Dr. Cortell who is working on an invisibility formula. Cortell dies shortly later and racketeers come after the formula, kidnapping Cortell's children Nancy, Jack and Jerry. Wade becomes the Invisible Terror to rescue them. By adventure's end, he claims to turn the formula over to the U.S. government, as well as continuing as the Invisible Terror. Like other strips in the first issue of this book, it was used to advertise clothing, an interesting concept for an invisible man.
Iron Ace: 1942, Air Fighters Comics #2 (Hillman). In France, the warrior called "Iron Ace" fought alongside Charlemagne. After the Iron Ace's death, Charlemagne predicts that the Iron Ace would one day return as "the champion of freedom." Flash forward to WWII, British pilot named Captain Britain is shot down in the skies over France and hides in the castle of Dr. LaFarge, leader of a resistance group where the armor of the Iron Ace now resides. Germans track Britain to the castle and kill LaFarge. Captain Britain puts on the old armor and defeats the Germans in the castle with the Ace's sword. Inspired by the bullet proof armor, he makes himself a plane that becomes armored with a flick of a switch and uses it and the armor to continue his fight in the War..
 

Iron Gates: 1941, Captain Aero #1 (Holyoke). Wealthy Gordon "Iron" Gates uses his knowledge of science to solve crimes.

  Iron Ghost: 1941, Shadow Comics v1n10 (Street & Smith). The Iron Ghost is a humanoid robot/android invented by Frank Reed. His body is composed of a super metal that is not only bulletproof but can resist the heat of the Earth's core, although he is otherwise subject to metal dissolving ray guns or stunned by ordinary ray guns. He can pass for a normal human, but his metallic skin is colored brown, leading me at first to mistake him as a possible minority superhero. Issues 10 & 11 are his only two appearances I know of. NOTE: The name of Frank Reed is possibly an intentional play on the name of the character Frank Reade, Jr. Reade was a fairly famous edisonade (boy inventor fiction popularized in dime novels) half a century earlier, being the inventor of the famous "Steam Man of the Praries", one of the more famous stories of its type.

Iron Lady: 1947, Airboy V4 N1 (Hillman). Doris Parker fought crime in an evening gown and a pair of iron gloves. Her gloves were made by a little Swiss watchmaker who got tired of his delicate hands being crushed in handshakes with big brutes. They are mechanical in nature, allowing Doris to be able to easily mangle a gun.

  Iron Man: 1948, Dick Tracy Monthly #1 (Dell). Detective Jim O'Brien is a tough as nails cop who prefers to work alone and this has earned him the name "the Iron Man". He likes to patrol in the disguise of a bum. NOTE: The one story I've read, I didn't see anyone actually calling him that. Drawn by Bill Ely.
  Iron Munro: 1940, Shadow Comics #1 (Street & Smith). Iron Munro travels from planet to planet fighting evil. He is aided in his endeavors by his friends Spencer Carlisle and Anto Raul as well as seeming to have invulnerability and super-strength. NOTE: The character is loosely based on character Aarn Munro from short-stories by Joseph W. Campbell. Later the name was re-cycled by Roy Thomas as Arn "Iron" Munro in YOUNG ALL-STARS.

Iron Skull: 1939, Amazing Man Comics #5 (Centaur). Iron Skull was a super-powered individualwho fought crime and the Axis. He is superstrong and bullet-proof but is sometimes mentioned at being vulnerable in the "main artery in his left forearm." Over time, details concerning the Iron Skull changed. In the beginning, he was more pulpish, given to wearing disguises and marking crooks with a skull ring. It also seemed that only his head was bullet-proof although he also possessed super-strength and his stories definitely seemed to be contemporary to the publishing date but issue #7 gave his origin at taking place in 1950 (and his adventuring career around 1960 and into 1970). He also carries a device up his sleeve called an Annod-Comptod Machine that sends out electric rays but is used sparingly. After several issues, the conceit of the adventures taking place in the near future is dropped and he seems to be firmly in the 1940's without an explanation. During one case his whole body is made steel hard by a villain, meant to immobilize him but just made him tougher. Later, he'd also wear a skimpy costume, trying to look a little more super-heroic. Most readers seem to assume he's an android or robot because the creator of the strip Carl Burgos also created the original Human Torch. However, in issue #7 we get an origin that says WWII waged on into the 1950 where an un-named soldier in Chicago is so battered and smashed, that Dr. Watson replaces much of his flesh and sinew with steel and iron. A decade later during a time of Reconstruction, the unknown soldier arises to fight crime as the Iron Skull (or just the Skull). Which means he's more of a cyborg than android or robot.
Iron Vic: 1940, Single Series #22 (United). A dying amnesiac found on the beach is treated by Dr. Degnan and Professor Carvel who had developed a serum that when treated with infra-red rays would grant great strength and intellect. Carvel dies soon after. His past a mystery, Vic becomes a mystery man aided by Dr. Degnan. Vic does his bit in fedora and fancy suit and cape but no mask.