Jack and Jill. Jack and Jill Doe are a married pair of wise-cracking amateur detectives. Think Nick and Nora Charles, but in the comics and not as good.
First Appearance: Hit Comics #1 (Quality)

Jack Andrews. Jack is a resolute and square-jawed high school student and four-star athlete who gets himself into lots of trouble fighting crime and criminals.
First Appearance: New Fun Comics #1 (DC)

Jack Dale. "Flying Cadet Jack Dale" is an Army Air Force cadet at Randolph Field, Texas--"the West Point of the Air"--who helps save fellow pilots during test runs. He then goes on to have various aerial adventures while still a Cadet.
First Appearance: Air Fighters Comics #1 (Hillman)

Jack Dewey. Jack Dewey is the youthful (12 years old, maybe) midshipman on the Hornet, a sloop cruising along the United States coast soon after the Revolutionary War. As is the way of things, through stout heart and strong arm Jack succeeds and begins making his way up the chain of command.
First Appearance: New Fun Comics #2 (DC)

Jack Frost. Jack Frost is a native of the Arctic. He finds a gold prospector dying of a gunshot wound, and goes to New York City to avenge the prospector's death. He does so, saving the prospector's daughter in the process but making himself an outlaw. Despite this he goes on to fight crime and evil. He can generate cold and project icicles.
First Appearance: USA Comics #1 (Timely)

Jackie Law.  Saith Ronald Byrd:

Dressed in matching t-shirts, Jackie's peers included Buck, Corny, Froggy, and Gorilla (only the latter was given much of a personality; a scrawny pipsqueak of a lad, Gorilla was the Boy Ranger always most eager for a fight (it's irony, get it? get it?)).  Although commanding the hollow robot-vehicle Loco in one adventure (a Ranger in each limb and Jackie in the torso, predating the Power Rangers by a good five decades or so), the Boy Rangers were primarily just the usual band of typical pre-teen boys who could easily outfight a band of hardened criminals or invading Nazis, leading one to wonder why the juvenile delinquent problem wasn't solved simply by shipping any troublesome young blades overseas.

Jack of Spades. An innocent man is murdered in a gambling den. His last card, the Jack of Spades, falls from his hand and comes to life as it hits the floor. Jack swears to avenge the man and to rid the entire world of crime. He then "returns to his paper form, where he may always be found in any deck of cards awaiting the call of mystery and adventure in his battle against the forces of evil." He can also fly and he has a very stretchable cloak.
First Appearance: Tops Comics #2001 (Lev Gleason)

Jack Terry. Captain Jack Terry is an American Army Air Force pilot who fights against saboteurs and all things German. He's a two-fisted hero of the usual sort.
First Appearance: All Great Comics #1 (Fox)

Jack Wander. Jack Wander is an American reporter and war correspondent sent to Europe to cover the war. His ship is sunk by a U-boat and he is taken prisoner by the sub's crew, only to be freed when the U-boat is bombed by a British plane. After all of that Jack makes it to Britain and begins doing his job. However, he redefines the parameters of his job so that it includes actively opposing the Germans, investigating and exposing spies, and in general making the news rather than just covering it. He is helped by his best friend George Melton.
First Appearance: Super Comics #21 (Dell)

Jack Woods. Jack is a do-gooding cowboy who fights for good and against them durn Mexicans down in Nogales. He has no superpowers but, like all the other comic book cowboys, is good in a fight and with his guns.
First Appearance: New Fun Comics #1 (DC)

Jane Drake. "Sheldon Drake, prominent attorney, finds that his daughter, Jane, much to his discontent, gets herself involved in cases that come through his office. Jane's secret ambition is to be a woman detective. Her chief aid and best boy friend is Jerry King, who lives next door. He isn't much interested in Jane's plans to wipe out crime. However, despite his many protests he usually gets himself involved iwth Jane in her many close calls."
First Appearance: Crash Comics #1 (Tem Publishing)

Jane Martin. Jane Martin, the "War Nurse" (not to be confused with the War Nurse) breaks hearts, heals breaks, and spies on the Germans from London.
First Appearance: Wings Comics #1 (Fiction House)

Jap-Buster Johnson. Lieutenant Doug Johnson, a native Bostonian, is better known as "Jap Buster Johnson," one of the most bloodthirsty of Marvel's WW2 military characters. He is serving as a U.S. Naval Air Force officer on a U.S. aircraft carrier "steaming off the Hawaiian Islands" when his carrier is strafed by a Japanese Zero fighter. His best friend Dave Nichols is killed during the attack, and in retaliation he hops in his plane and goes on to wipe out a wing of Zeroes. Not content with that, he hops in a Zero in a Japanese uniform, goes to a secret Japanese island base somewhere in the Pacific, infiltrates it and calls in an air raid to destroy it. And that's only in his first appearance. The rest of his episodes (the ones I've read, anyhow) are pretty much like that--lots of action with a high body count across the Pacific, with Johnson continually invoking Dave's name.
Notes: I realize that using the word "Jap" to describe someone of Japanese heritage/descent/ethnicity isn't cool, but that's the name of the character. The strip itself was horribly stereotypical, with bucktoothed, squint-eyed Japanese soldiers, but that's wartime propaganda for you.
First Appearance: USA Comics #6 (Timely)

Jaxon. "Jaxon of the Jungle" is Mike Jaxon, "adventurer and expert jungle guide," who is active in Africa doing the Great White Hunter/Explorer thing..
First Appearance: Prize Comics #1 (Prize)

Jeff Barter. Jeff Barter is a trader in Africa, who with his assistant Ted Collins tries to make a profit while fighting evil and helping the "good" natives against the "bad" natives.
First Appearance: Bang-Up #1 (Progressive)

Jeff Dix. Jeff Dix is better known as "Corporal Dix" or "Sergeant Dix," depending on the issue. He is "the tough guy of Uncle Sam's army," a two-fisted patriot who fights for good on the homefront, whether at his parents' house, on the Army base, or while on leave in the big city. He never seems to make it to the frontlines in the stories I've read with him, although he does get transferred to the Panama Canal in one story, where he and his (stereotypical) African-American best friend Fish-Face Friday fight off an attack by Japanese bombers. In most
stories he is given a special assignment to root out Fifth Columnists, which causes him to go undercover in wood pulp factories and other
places to discover the evil German menace.
First Appearance: USA Comics  #4 (Timely)

Jester. Chuck Lane is a rookie cop on the NYC force who tends to make the other cops laugh through his own clumsiness. Chuck decides to capitalize on this by dressing up in a jester outfit and fight crime as The Jester. He had no superpowers, but was agile and a good fighter.
First Appearance: Smash Comics #22 (Quality)

Jim Dawson. Dawson is a soldier of fortune active in the Indies.
First Appearance: Wonder Comics #1 (Better)

Jim Dolan. Dolan is a two-fisted crime-fighting magazine editor. He's a former F.B.I. agent and is the editor of Daring Detective Magazine who "goes out after unsolved cases--solves them and brings back the story."
First Appearance: Slam-Bang Comics #1 (Fawcett)

Jim Giant. Jim Giant is "the strongest man in the universe." In the future "hordes of savage warriors" sweep across the world, using advanced technology and "deadly ray guns" to attack civilization. The big Jim Giant, who is about ten feet tall, gets involved (at the urging of "his good friend, a scientist." Naturally, Jim has great superstrength and quickly destroys the invasion.
First Appearance: Planet Comics #4 (Fiction House)

Jim Hatfield. Jim Hatfield is a Texas Ranger active in the Old West.
First Appearance: Exciting Comics #1 (Better Publications)

Jimmy Jupiter. Jimmy Jupiter is a child who has the ability to travel to, in the words of the indefatiguable Ronald Byrd,

     Little Nemoish fantasy-lands inhabited by intelligent insects, talking clocks, fairies, demons, and the like.  His two friends from
     "the Land of Nowhere" were the dragon Wump-Jump and Ruffy Rabbit, both of whom materialized in the "real world" in one
     story.

He is from the real world and is based here, and when a group of bullies picks on a new kid Jimmy comes ot the new kid's aid. The people
of Fairyland, in one issue, note that "in each generation there is one who can visit" their world, which explains, I guess, Jimmy's power.
First Appearance: Marvel Mystery Comics #28 (Timely)

Jinx. Darrel Creig is the 16-year-old sidekick of 13. He is very lucky, like 13, although it's unclear whether this is a superpower or not.
First Appearance: Daredevil Comics #5 (Lev Gleason)

Joan Mason. Joan Mason is "the country's leading girl reporter." She's beautiful and clever and works for the Daily Planet. (!) She is helped by police officer Mike Mannigan, but it is Joan rather than Mike who is usually responsible for bringing in the bad guys. She provides the brain, he provides the brawn.
First Appearance: All Great Comics #1 (Fox)

Jo-Jo. Jo-Jo is a Tarzan clone who keeps the peace in the jungle with the help of his girlfriend Tanee.
First Appearance: Jo-Jo Comics #7 (Fox)

John Law. John grows up on the wrong side of the tracks--"Crossroad City"'s Lower East Side. His father is a widower, a worn-out pug and lowlife crook. John's father accidentally kills his best friend, Officer Serf, during a robbery, and the kills himself. John goes to work as a clerk in the D.A.'s office and then joins the police force, fighting crime and implicitly stopping people from becoming like his father. He has no superpowers, but is "strong, decent, and believes in the rules he defends." He has no superpowers and is missing his left eye.
First Appearance: Smash Comics #3 (Quality)

John Steele. During the early days of World War Two "Soldier of Fortune" John Steele fights for the the Allies in the European front lines. He's very strong, capable of knocking a man several feet into the air with one blow and ripping a steel hatch open with his bare hands (his strength is arguably superhuman), and is a capable hand-to-hand fighter. He's also relentless, not willing to let a little thing like exploding grenades or dive-bombing Stukas slow him down.
First Appearance: Daring Mystery Comics #1 (Timely)

Johnnie Law. Johnnie is a tough rookie cop whose beat is the "poor tenement district of the Lower East Side."
First Appearance: More Fun Comics #21 (DC)

Johnny Canuck. Johnny is one of Canada's foremost heroes, a top pilot, Air Force Captain, and secret agent both on the front lines of WWII and in occupied territory. He has no superpowers but is a very tough fighter and an excellent pilot.
First Appearance: Dime Comics #1 (Bell)

Johnny Peril. Johnny Peril is an adventurer of a sort, although his exact profession—private eye? Reporter? World-weary existentialist?—is never made clear. Nor is his background and personal history. Suffice it to say that he’s an adventurer who tangles with nearly every sort of enemy in nearly every sort of background, from the jungles of the Congo to the concrete jungles of New York to the moon. He has no superpowers, but he’s tough, clever, and skilled, and good enough with knife and gun that he never needs powers.
First Appearance: Comic Cavalcade #22 (DC)

Johnny Quick. Johnny Chambers was taught by Professor Gill, his legal guardian, a mathematical formula, "3X2(9YZ)4A," which when uttered bestow superspeed. Chambers used this power to fight crime as the heroic Johnny Quick. In his civilian identity he is a cameraman for a newspaper. He is assisted by his portly sidekick and stereotype Tubby Watts. His superpower is superspeed, which also allows him to fly.
First Appearance: More Fun Comics #71 (DC)

Johnny Rebel (I). Johnny Bailey, a blond teenager, is "Johnny Rebel," a fighter for good in the Antebellum South. He is assisted by a racist stereotype, the black slave Rufus.
First Appearance: All New Comics #1 (Family/Harvey)

Johnny Rebel (II). Jakeoster contributes the following:

Johnny Rebel fought crime in the southern states wearing a costume given to him by his grandfather Bailey. He had no
super powers.

Johnny Thunder (I). Johnny Thunder, born on the seventh day of the seventh month of 1917 at seven A.M., is given the power of mastery over a pink lightning bolt, which obeys his wishes and is extremely powerful. In his civilian identity he is a moron. He is a member of the Justice Society of America which was clearly in need of comic relief when they took him onboard. His superpower is to command the lightning bolt, which is a kind of genie which will do anything when commanded by the magic words, "Cei-u."
First Appearance: Flash Comics #1 (DC)

Johnny Thunder (II). John Tane was forced, by his dying mother, to promise never to use a gun and instead to become a schoolteacher. All well and good, but when he tries to teach peace, rather than enforce it, he is looked down upon by his father, who wanted him to be a shootist. So John puts on make-up and a costume and fights crime as Johnny Thunder, the only law west of the Pecos. He is helped by Black Lightning, the wonder horse. Johnny Thunder has no superpowers but is really good with his guns.
First Appearance: All-American Comics #100 (DC)

Jon Linton. Jon Linton is a "flyer, scientist, adventurer" who fights against the evil alien Nogos in the year 2000 on "Quinton, Land of the 5th Dimension." Jon does this with the help of his friends Dr. Kane, Lisa Kane (Jon's sweetie), and Alpha-712, a native of Quinton who can "make from mental force anything they have seen or understand." After beating the Nogos Jon et al travel back in time and arrive on Earth in the year 1940.
First Appearance: Wham Comics #1 (Centaur)

Judge. Jim, of no last name, has to watch as his father is killed and then the murderer, due to scared witnesses, goes free. Jim devotes himself to the law, wanting to avenge his father. He becomes a judge, and a hangin’ one, but when the law is insufficient he puts on a mask and takes a gavel to crooks as the Judge.
First Appearance: Red Circle Comics #1 (Rural Home)

Judy of the Jungle. She is a Sheena clone with a pet black panther named Kala and a Great White Hunter boyfriend named Pistol Roberts.
First Appearance: Exciting Comics #55 (Standard)

Jun-Gal. "Rising from the dark recesses of Equatorial Africa is a legend of Jun-Gal, white goddess of a native tribe who guards THE PIT OF DEATH!" She's actually the daughter of Professor John Teal and was raised by a native tribe after Teal's death. Unfortunately, the strip is more than usually racist for this sort of thing and I don't have the stomach to continue reading it.
First Appearance: Blazing Comics #1 (Enwill Publishing)

Jungle Boy. A white boy raised in the jungle Tarzan-style. His real name is Bomba.
First Appearance: New Comics #4 (DC)

Jungle King. Lee Granger, a scientist-explorer, is the Jungle King, a Tarzan-type accompanied by Eric, an intelligent, English-speaking lion. (Granger taught Eric how to speak English) He's known as the Jungle King for his good deeds and for "organizing a pygmy community along modern lines."
First Appearance: Slam-Bang Comics #1 (Fawcett)

Jungleman. Jungleman is a Tarzan type accompanied by Keeta, an albino tiger, and operating in the jungles around Angkor Wat.
First Appearance: Champion Comics #2 (Harvey)

Jungle Twins. Bill and Steve Dale are the Jungle Twins, Tarzan types fighting crime and evil in the jungle.
First Appearance: Nickel Comics #1 (Fawcett)

Junior Rangers. The Junior Rangers were Smokey, Roger, Chan, and a few others. These were kids of various ethnicities who came together to fight crime and do good.
First Appearance: Headline Comics #1 (Prize)

Jupiter. "Jupiter is sent from his planet to Earth to clean up the evil corruption." He is "the master magician" and can perform all sorts of feats with his magic, which is not of the backwards-speaking variety.
First Appearance: Prize Comics #1 (Prize)

Justice Society of America. First and greatest of all the Golden Age superteams, the Justice Society of America, or JSA, was formed first by a group of heroes sitting around shooting the bull, and later formed to fight threats. The JSA's membership consisted of the Atom, Batman, Dr. Fate, Dr. Mid-Nite, the Flash, Green Lantern, Hawkman, Hourman, Johnny Thunder, Mr. Terrific, Red Tornado, Robin, Sandman, Spectre, Superman, Wildcat, and Wonder Woman.
First Appearance: All-Star Comics #3 (DC)

Just 'n' Right. Jakeoster contributes the following:

Justin Wright was a lumberjack who parents had been murdered. He puts on a blue business suit and ties his mother’s scarf
over his eyes and vows to fight crime. He had great strength, but no super powers.

Introduction
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