Jack the Clipper: 1945, Topsy-Turvy Comics #1 (R. B. Leffingwell and Co). Anthony Durrant writes: Jack the Clipper was a maniac who clipped off the beard of the famous Civil War veteran Admiral Rockjaw, and later cut off the long white hair of a United States Senator. He was caught by the boy detectives Mickey and Ickey and unmasked as their favourite cartoonist, J. Drawingboard Easel. Easel had been running out of ideas and had adopted the nom du guerre Jack the Clipper in order to gather material for his comic strip, Subman.
Jack the Rabbit: Champ Comics 22 (Harvey): Recurring foe of the Liberty Lads: Skip and Chuck, two Simon & Kirby boy types who seek out excitement accompanied by G-man Lee Hunt. Jack was a round fella with pointed ears, loud clashing clothes and not above gruesome torture.
The Jackal: 1944, Green Hornet Comics #17 (Harvey). Dressed in a canine outfit, the Jackal looks like a human-sized version of his namesake. He embarks on murderous acts of sabotage but is stopped and unmasked by the Zebra. Turns out he's a timid weak looking clerk in the shipyard office pushed to murderous rage by the continued bullying and taunting of the burly shipyard workers.
Jackel: 1940, Silver Streak Comics #7 (Lev Gleason). "Fat Sam" Jackel runs an airline and at the behest of the exotic Euroasian beauty White Dragon Flower, plans on replacing several pilots of a General Staff flight with those of her spies in order to kidnap the officials. Stopped by Cloud Curtis.
Jake & Lil: 1948, Dynamic Comics 24 (Superior Publishers). Billed as the king and queen of the silver wire (high wire), this couple employed a large rubber ball that Lil would burst out of during their act. She would then give a small souvenir ball to someone in the audience. Usually this person was the kid of someone they planned to rob. At home under electric lights, the ball would expand and burst, letting loose a gas that would kill everyone in the house. Then Jake and Lil would pilfer the house at their leisure. Unfortunately for them, Dynamic Man figured out their scheme.
Japanese Sandman: American bomber pilots are executed by someone calling himself the Japanese Sandman and sends word back to the Americans. Captain Raleigh aka the Boomerang flies to Japan in disguise to get to the bottom of the mystery. What he finds is a wily Japanese Colonel and some Japanese men schooled in the arts of Jiu Jitsu and hand to hand combat, the best of whom is the Japanese Sandman. Thanks to the timely arrival of his girlfriend Diana in a costume near identical to his and armed with a bow and arrows, Boomerang wins the day.
'Faces' Jeffre: 1940, Amazing Mystery Funnies v3 #1 (Centuar). Faces had been a noted actor in vaudeville, able to to easily change his voice, face and mannerisms to suit his roles. However, when vaudeville dried up, so did the the demands for him as an actor and he turned to the easy life of crime. He quickly discovered that his abilities on stage gave him a great advantage, to be almost unrecognizeable. Eventually, his crimes and boldness grew to the point that he wasn't above committing murder and he was captured, tried and sentenced. He used his acting ability to escape prison and met back up with his gang. He didn't stay free very long as he opted to disguise himself as a local farmer while shopping at a store in New Jersey. He gave himself away as his acting seems to have been more of mimicry than method, so he bought more stuff than a down and out farmer would have, didn't bother to disguise his decidedly non-farmer hands and the clincher, after giving exact change for everything, he did the same for a pack of cigarettes - not realizing that in New Jersey, they didn't charge tax for cigarettes like they do in New York. A small text story, that doesn't have him facing any super detectives or such, but his skill at disguise made him notable enough.
The Jester: February, 1949, Western Adventures (A.A. Wynn): This murdering owlhoot and his gang were raiding local gold-mines as well as Wells-Fargo stages forcing townspeople to go to Jake Bellum for extravagent loans. Even Bellum was a bit of a mystery man, who kept his face in shadows while making his deals. Things were good for the Jester and Bellum until the Cross-Draw Kid interrupted one of his gold heists. He soon tracked the wily thief to the office of Bellum and discovered Bellum was nothing but a stuffed dummy used by the Jester. Following his trail, the Cross-Draw Kid revealed to the townspeople that the Jester (and Bellum) were aliases for "Lips" Hawkins, a ventriloquist who performed at the saloon. Chasing the Jester, the Kid uncovered the fact that the big gang that the villain had used to terrorize the small town was also nothing more than dummies on horseback. After a big fight, the Kid took the Jester back to town to face justice.
Jig and Blackie: 1936, Funny Picture Stories v1n2 (Comics Magazine Company/Centaur). Jig and Blackie are smugglers. Jig seems to have somewhat bug eyes, they are often wide open. He's a mean guy and has a foul mouth. Blackie is his moll, short dark hair and sultry. They are captured after they take on a customs agent "Free" Smith as muscle.
The Jingle Man: This killer sent rhyming letters to his future victims, signed the Jingle Man. He was captured by the Scarlet Archer on his first case revealing him as the Archer's romantic rival, bitter at the death of his father blamed on a group of financial schemers who had forced him out of the group. He targeted each man (and the girlfriend since her father died years before) and killed them in keeping with their specialties: finances, food supplies, etc.
Joker: 1946, Young King Cole (Novelty). Dennis Durrant writes: Not the Batman villain, but the leader of the Spade Gang, a group of criminals whose codenames are the Ace, the Deuce, Three, Four, Five, Six, Seven, Eight, Nine, Ten, King, Queen and Joker - the hooded leader of the gang. The Spades went up against the detective agency led by Kingston Cole senior, the uncle of Dick Cole the Wonder Boy and the father of Kingston Cole junior, better known as Young King Cole. The original Spades were captured and jailed by Kingston Cole, but the Joker remained at large and formed a new Spade Gang, then went after the Cole Agency's secret files, in the process killing Kingston Cole and leaving Young King Cole in charge of the agency. Eventually, Young King Cole saw to it that the new Spades were captured and killed, then unmasked the Joker as a woman, apparently the widow of the original Joker. She drove her car off a cliff and Young King Cole just barely was thrown clear of the wreck alive. The Joker first appeared in Young King Cole #3.
Joltin' Joe: 1945, Star Studded Comics #1 (Cambridge House). This bug-eyed crook was insuring packages that were being shipped, and then sinking the ships in order to collect on the insurance. Turns out he was in league with the insurer who was milking his own company, defrauding the stockholders. Both discovered it was very unlucky to cross Luckyman.
Josef: Gordon Fife and the Boy King (newspaper strip?) The large Sydney Greenstreet type villain headed a criminal gang in Kovnia. He was a little unique in that he used hypnotism to hypnotise Gordon Fife to steal the crown jewels.
Juggernaut of Destruction: 1943, Green Hornet Comics #14 (Harvey). This giant crawled forth from a smoking volcano, caused vast destruction, ate Reid's gal Miss Case before returning to his volcanic lair. Following him, Green Hornet and Kato discover that the volcano is only made to appear active to mask the activity of a vast laboratory and headquarters of Nazi saboteurs. Likewise, the giant is merely a giant machine operated by agents inside it where they are holding the kidnapped Miss Case. The giant's size is described as being 6 stories tall, but the art is a little inconsistent displaying it. The name comes from the title of the story. Internally, he's only referred to unimaginatively as "the monster".
Juke Box John: 1942, Four Favorites #6 (Ace). Juke Box John runs a diner of the same name near a construction plant. He forces Arlene Clark, an escapee of a woman's reformatory to steal plant secrets or he'll continue killing workers through his unique method: he fakes a phone call for a worker when they are in the diner and the phone squirts a special liquid deep into their easr. Then he has a particular record on the juke box play that hits a specific frequency causing the atoms of the liquid to explode, destroying the victim's brain. Magno and Davey expose Juke Box John as being Arlene Clark's father John Clark who owned a rival company that would have gotten the contract if the company couldn't finish on time. When Arlene got wind of it, he originally had her framed and sent to the reformatory and then blackmailed her into helping when she escaped. Magno seemed more than a little smitten with Arlene through the course of the story.