Mystery Men & Women:

The N's

Nature Boy
 Nature Boy: 1956, Nature Boy #3 (Chalrton). The youth David Crandall is lost at sea but instead of drowning, he's rescued by various gods who give him the powers to command the elements (much like Aquaman is able to command fish). There is also a Nature Girl and Nature Man, who is possibly David grown up, ala the relationship between Superman and Superboy (whom Siegel had originally proposed to DC, and was turned down only to have the lad of steel introduced a while later independent of him and his heirs had to sue for that recognition). NOTE: A little late for the intended scope of this site (after J'onn J'onzz but before the Flash), but the character is largely unknown these days and it features some of the earliest, if not THE earliest known artwork of John Buscema and co-created by Jerry Siegel.

Navy Jones: 1940, Science Comics, #1 (Fox). Descendent of legendary Davy Jones, Navy Jones gains the ability to breathe underwater and forsakes the land to live undersea, helping out a technologically advanced race of fishmen, and their human looking king and daughter Princess Coral as well as other undersea races. During the course of his career, he even teams up with Captain Nemo!

Like many of the heroes of Science Comics, his costume would undergo color changes from issue to issue. In addition to breathing underwater, Navy is able to withstand the pressures of the deeps and has some degree of super-strength judging from how he swings a heavy anchor around! The non-human fishmen are pictured below.
  Johnny Nebisco: 1945, Front Page Comic Book #1 (Harvey). Smart, tough and quick talking Johnny Nebisco (also called Johnny the Nebisco) is a police detective. He constantly mouths off to his boss the Commissioner and seems to exist in a state of either quitting or getting fired due to his attitude, but it seems to be just all talk between the two.
  Dr. Neff: 1948 Red Dragon Comics #3 (Street & Smith). Bill Neff is a ghost breaker and fought Professor Drade, the zombie master. NOTE: Bill Neff is one of those real life magicians like Blackstone who had fictional comic stories built around him. Billed as "The Master of Many Haunts", he held midnight spook shows where he called forth all sorts of ghosts, ghouls and apparitions.
Nelvana
 Nelvana: 1941, Triumph Adventure Comics #1 (Hillborough Studios/Canada). Daughter of the god King Koliak, the goddess Nelvana is a protector of the native peoples of Canada meaning she fights their enemies whether they be mystical or the more Earthly Nazi menaces. She is helped by her brother Tanero. The pair have various god-like powers: able to disguise themselves as mortals, turn invisible, travel at super-speeds. NOTE: John Byrne intended for Snowbird of Alpha Flight to be the daughter of Nelvana
  Lt Nick Nelson: 1940, Cyclone Comics #1 (Holyoke). Nick is a lieutenant of the US Navy and commander of a Q-Boat.
Neptina: Champion Comics #2 (Harvey). Neptina is the queen of Amloza and its fish-men, an underwater race. She starts out villainous, eager to conquer the surface world and kidnap surface people in order to transplant their lungs to her and subjects to allow them to breathe as easily on land as water. She is first opposed by Lt Fletcher of the US Nay and his girlfriend Norma Kane. She is overthrown by her sister Mhera and flees with her right-hand fishman Mogg. Mhera proves to be an even worse ruler and Neptina reforms so Fletcher and Co. seek to restore her to the throne. For awhile, she is jealous of Fletcher's girlfriend, the capable Norma Kane but eventually Neptina falls for the dark-haired Prince Eon who has been overthrown in his own underwater kingdom Polaris by King Walro and the Walrus men. Don't know why her, her sister, and Eon look human despite being able to breathe underwater while their subjects decidedly don't.
  Doctor Nerod: 1940, Green Giant #1 (Pelican). Nerod is a youngish man and an incredible scientist working in a remote mountain lab. He comes up with a way to stop war by releasing flies that carry a germ of a sleeping sickness that will cause bite victims to slip in a coma after an hour and within two months, die unless given an antidote by Nerod. The flies can be kept under control by being destroyed by an ultra- or infra red ray (called both in the story). Enemy troops are given warning and then the flies released. Soldiers that sign a peace pact after being restored are released, enemy leaders are treated while in prison. Any who sought escape and not found would die in two months.
Night Bird: 1941, Green Mask #6 (Fox). Newspaper photographer "Lens" Crockett is secretly the non-powered mysteryman Night Bird. He frequently is paired up with lady reporter Dell Daniels who doesn't think too much of Crockett.
  Night Devils: 1942, War Stories #5 (Dell). American Lt. Dan Cork and British Lt. Tom Harrow are the black clad Night Devils. The strip is most likely a continuation of the characters Black Wings in Dell's War Comics #4,, though there they were known only by their code numbers.
Night Hawk
 Night Hawk:1942, Dynamic Comics #3 /1943, All New Comics #1 (Family/Harvey). Jack Filan is one of the bored playboy turned mystery men types. His fiance is Jane Curry. In the one story I've seen, he wears a blue costume (not gray as here) with green trunks and boots and his top is actually loose fitting. He's acrobatic and a good fighter but otherwise non-powered.. He had a very striking foe in the Green Skull. NOTE: It's interesting to note that the character and villain seem to have first appeared in a text story at Chesler.

Nightmare and Sleepy

 Nightmare: 1943, Clue Comics #1 (Hillman). Wrestler Terry Wake dresses up in a skeleton costume to fight crime alongside his kid manager as Nightmare and Sleepy. Early on, it was clearly that the skeleton and skull and crossbone on their costumes glowed in the dark, further spooking their foes. They fought oddball criminals, one of the better Batman & Robin/Sandman & Sandy type duos. He later had a more boring and generic costume. In his next to last appearance, he appeared without Sleepy and would emerge from the smoke of a cop's cheap cigars, but would only stay around as long as the cigar stayed lit.
Nightro: 1941, Daredevil #2 (Lev Gleason). When Hugh Goddard and his comrades Hoag and Tollini discover a rich radium mine in the wilds of Alaska, Goddard wants to give it to the Cancer Cure Foundation and of course they want to get rich. Beaten up and left for dead, he wanders through a vicious snowstorm. Before he completely falls to exhaustion and snow blindness, he's discovered by scouting eskimoes. Even as he physically recovers, his blindness remains. Examined by Dr. Frank Miller, he's told the condition is permanent but has a unique quality, with dark poloroid glass in front of his eyes, he can see perfectly, day or night. Equipped with goggles, he makes a costume to match and becomes the “Streamlined Robinhood” Nightro. First order of business, bring his treacherous partners to justice. He continues to fight crime and is sometimes aided by his brown guide dog Blackie. Wow. A comic named Daredevil, someone named “Frank Miller” and a hero who's blind but whose origin includes radiation. Can't make this stuff up.
Nightshade: 1941, Amazing-Man Comics #24 (Centaur). Howard Hall has the ability to control his shadow so he straps small flashlights to his ankles and when turned on, he has his shadow. While the shadow can go anywhere a shadow normally could, he can eavesdrop on coversations through it and it can be solid enough to punch out a crook. To control the shadow, he must pantomine the actions he wants the shadow to perform.
  Tom Niles, the Undersea Raider: 1940, Thrilling Comics #2 (Standard). Tom Niles is a submariner and Commander for the US Navy and takes on special missions. After a successful mission, he is given command of the submarine blockade squadron and Joan Hawley of the British Intelligence is to serve as his espionage agent. Other than ably captaining submarines, Tom Niles is good at diving, disguise and two-fisted action when the situation calls for it.
  Dr. Nils: 1939, Keen Detective Funnies v2#12 (Centaur). Right after he sends his son to college to study medicine, plastic surgeon to the rich and famous Dr. Nils is taken by gangsters to change the looks of notorious gangster Torske under threat of violence to his son. Even when the police burst in and the papers subsequent tar and feather him as an accomplice, he lets his business slide into ruin rather than endanger his son. When he has to treat Torske again, he's paid in cash and pays his nurse. Sadly, the bills are marked from a recent hold-up and Nils is arrested this time. His son is graduating when he hears of his father committing suicide by jumping in a river while on bail. The young Nils follows the trail of Troske and his new face. Only he finds an old bearded tramp everywhere he goes. And when Troske with his new face arrives, the tramp spikes Troske's drink and kills him. Then the tramp reveals himself as the older Nils, having brought Troske to justice "dead or alive" as the police wanted. He promises to start his life anew, only now with a new assistant. Presumably, Dr. Nils, father and son, kept their noses clean after this.
  Nordac: 1941, Double Comics (Gilbert). A huge nordic giant, looking a bit like Thor bashes the Nazis with his hammer. More than likely his adventure is solely on the cover as these comics were often re-packaged reprints, with a few of them having cover-only stars.
  Lt. Larry North: 1941, Exciting Comics #10 (Better). Larry North of the US Navy undertakes special cases. Like most heroes, he's good at flying a plane, a crack shot and a good fighter. He is helped by the pretty blond Betty Harmony. In issue #34, while stranded on an island, he and Betty meet up with Hal Thurston, a lad who had been doing the Tarzan bit and harrassing the Japanese to the point they dubbed him the "wild man of the jungle". At the end of the issue he sticks with Larry and Betty as he has no family of his own. The following issue was the last for North & company.
  Frosty North: 1940, Fight Comics #10 (Fiction). Frosty North is a Federal agent in Alaska. He sometimes teams up with the beautiful brunette agent X-7 of British Intelligence.
  Nickie Norton: 1940, Thrilling Comics #1 (Better). Nickie Norton is a Secret Service agent fighting the enemies of America. His superior officer is Inspector McGinnis.
  Numa: 1948, Rulah (Fox). Sheena type jungle queen down to the the strawberry blond hair. Numa does have a modern looking knife (and an expert at throwing it) as well apparently skilled in Judo.
Number 40: 1941, Our Flag Comics #3 (Ace). Twenty years earlier, Martin Benson was kidnapped as a baby, one of 200 by the Germany loving Von Monsta. He was raised, trained, and brainwashed to be a cruel brute and given an injection granting him great strength and vitality. However, he rebels against his master and comrades' cruelty and helps the Flag against them. A head injury clears his thinking and through a locket, the Flag is able to restore him to his parents.