X Bar X Boys. The Boys were created by Howard Garis and debuted in The X Bar X Boys on the Ranch (1926); they appeared in 22 novels in all, through 1943. The boys were Roy and Teddy Manley, sons of an "old ranchman" and very manly teenage brothers who were, in the words of one critic, "a reasonably accurate facsimile of the Hardy Boys, but moved out West and riveted firmly in the saddle."

The X Bar X Boys
A bibliography of the series.

The X Bar X Boys (II)
A nicely thorough site, with loads of information. A better effort than the X Bar X Boys deserve, frankly.

X37. X37 was created by Major A.J. Dawson and appeared in The Case Books of X37, a collection of short stories published in 1930. X37 is a retired British detective whose past is full of encounters with card sharpers, forgers, murderers, blackmailers, female thieves, and the like. His case books are full of accounts of life en bas, but nothing particularly exotic (or interesting).

Yankee Flier. The Flier was created by Thomson Burtis and appeared in the 16-volume "Air Combat Stories for Boys" series, starting with Daredevils of the Air (1932) and running through 1946. The first 7 volumes deal with Stan Williams, the titular flier, and his adventures flying fighters against the German air force during WW2. Before the war Williams had been an All-American halfback at the University of Colorado and an expert test pilot, but once the hostilities started he immediately volunteered for action. The last 9 volumes deal with Stan Wilson, an American pilot, who joined the Red Flight of the R.A.F. and fought, along with his best friends March Allison and Bill O'Malley, against the Axis forces over Occupied France, in Singapore (with the Flying Tigers), Tunisia, Italy, Germany, Normandy, and after the war against a German spy ring.

Red Flight
A good site on the WW2 Yankee Flier. From The Series Bookcase site.

Yankee Rangers. The Rangers were created by Alfred Andriola and appeared in an eponymous strip starting in 1942 and running for only six weeks. The Rangers--Chunky Tubbs, Rex Rand, and Riff Rafferty--were a group of commandos based in England and trained by Colonel Force to kill, kill, kill, kill!

Yen Sin, Doctor. The infamous Doctor Yen Sin was created by Donald Keyhoe and appeared in Dr. Yen Sin beginning in May 1936. Yen was essentially a Fu Manchu knock-off, working out of Washington, DC,  and making use of Burmese Dacoits, blow-gun assassins, torture chambers, mummies, the Shi-muh "Tibetan corpse-flowers" (poisonous puff-balls), and the usual Fu-inspired fun. Sin, aka "the Crime Emperor," aka "the Invisible Peril," had green eyes with enlarged black pupils. His symbol was the cobra. His ultimate goal is to bring about war between Japan and the U.S., with the aim of rising to power afterwards from the ruins of both countries. Yen was helped by Sonia Damitri, a young and beautiful Greek woman whose father was held prisoner by Yen; she hated Yen and helped his enemy, Michael Traile (see below), whenever she could.

Yen was opposed by Michael Traile, "the Man Who Never Slept." Traile had been the victim, as a teenager, of a bad brain operation by a sloppy Hindu surgeon. The surgeon had damaged Traile's mind so that he could no longer sleep. Thankfully for everyone, Traile had been taught by a Yogi to relax his body completely, even though his mind was awake, so that he would be rested, even without sleeping. He needed the rest; without it, he would become horribly weak. Traile made use of the extra hours of the night to train himself, not just in detection but in the use of exotic weapons, foreign languages, and anything else that might provide him with challenges. Traile was assisted in his war against Yen Sin by Eric Gordon, an agent for the Justice Department.

York, Anton. Anton York was created by “Eando Binder,” aka Earl Binder and Otto Binder, and appeared in a series of stories published in Thrilling Wonder Stories between 1937 and 1940. Anton York is the son of Matthew York, an American scientist who in the 19th century discovered an immortality formula. (It works through a combination of chemicals and cosmic radiation.) Anton is given the formula, as is his wife, Vera, and together the two adventure first on Earth, defeating men who steal the immortality formula, and then in the galaxy, where Anton and Vera fight and defeat first a group of insane, techonologically-advanced immortal aliens and then later an invasion from another universe.

Yorker, Dick. (No, I'm not talking about Dick York. No, stop it. I'm not, and that's all there is to it.) Dick Yorker (hush) was created by the Russian filmmaker and writer A.A. Litvinov and appeared in Na Raznykh Beregakh (On Different Shores, 1925). Yorker is an American working in the factories, shipyards, and oil fields of Baku who with his impressionable young Turkish friend Abbas helps create an international worker's movement, saves the beautiful Edith Braid from danger, and foils the plans of Mack Neil, the vile secret agent of the capitalist oppressors.

Young Aeroplane Scouts. The Scouts were created by Horace Porter and first appeared (as far as I can tell) in Our young aeroplane scouts in Germany; or, Winning the iron cross (1916). The "Our Young Aeroplane Scouts" series ran for seven more novels, through 1919. The Scouts were Henri Trouville, of an ancient and honorable French family, and Billy Barry, of Bangor, Maine, and together the pair ranged across the world, fighting crime and evil and the Hun in France, Belgium, Turkey, Russia, England, Italy, and the Balkans. The pair were whizzo pilots, shooting down a dozen of the enemy at one go.

Young Alaskans. The Young Alaskans (don't know their names) were created by Emerson Hough and appeared in the five-book "Young Alaskans Series," which ran from 1908 to 1922 and began with The Young Alaskans. The Alaskans were a group of boys who had adventures and fought crime from Alaska to Colorado and into Missouri.

Young Birdmen. The Birdmen (don't know their names) were created by Keith Russell and appeared in the three book "Young Birdmen Series," which ran from 1929 to 1930 and began with The Young Birdmen on the Wing, or, The Rescue at Greenley Island. The Birdmen were a group of teens who had their own planes and used them for adventures and crime-fighting, including rescuing stranded travelers, making sure the night mail made it from coast to coast, and exploring the jungles of the Amazon.

Young Continentals. The Continentals (don't know their names) were created by John McIntyre and appeared in the four-book "Young Continentals Series," which ran from 1909 to 1912 and began with The Young Continentals at Lexington. The Continentals were patriotic American boys who fought the British at Lexington, Bunker Hill, Trenton, and Monmouth.

Young Engineers. The Young Engineers were created by H. Irving Hancock and appeared in the six-book "Young Engineers" series, which began in 1912 with The Young Engineers in Colorado or At Railroad Building in Earnest. The young engineers were civil engineers who had various adventures in the course of their job, laying tracks on "man-killing quicksand," sinking mines in Nevada, fighting mine swindlers in Mexico, making "million-dollar" breakwaters in the Gulf of Mexico, and such-like.

Young Mineralogist. (Sounds like a soap opera title, doesn't it?) The Y.M. (don't know his name) was created by Edwin J. Houston and appeared in the five book "Young Mineralogist Series," which ran from 1910 to 1912 and began with A Chip Off the Old Block, or, At the Bottom of the Ladder. The Y.M. was a hard-working young mineralogist whose interests brought him to dig and excavate in places as varied as the deserts of the Southwest, the Grand Canyon, the gold fields of the Rockies, and Alaska.

Yukon King. King was created by Tom Dougall and George W. Trendle and appeared in "Challenge of the Yukon," a radio show that ran from 1938 to 1955. "Challenge of the Yukon" told the stories of the Royal Canadian Mounties' fight against crime and evil in the Great Canadian Northwest. The lead Mountie was Sergeant Preston (yes, that Sergeant Preston), who with Inspector Conrad and Jacques, the French guide, cut a swathe through crime, but the lead character of "Challenge of the Yukon" was Yukon King, one of the largest and perhaps most intelligent Huskies ever born. Yukon King "mauled bushwhackers and crooks, gnawed guns out of hands, and hauled down one villain while Preston polished off the other." King was strong. Sergeant Preston joined the Mounties to capture his father's killer and did that, becoming successful enough to earn his Sergeant's stripes. Yukon King, for his part, was a Husky puppy that had been abandoned and raised by a female wolf, Three Toes. Preston came upon a lynx attacking Three Toes and King and saved King but was unable to save Three Toes. Preston continued raising the puppy King, who became devoted to him and was of course very intelligent.

Zapt, Dr. Xenophon. Dr. Xenophon Xerxes "Unknown Quantity" Zapt was created by J. U. Giesy (creator of Semi-Dual) and appeared in All-Story and Weird Tales from 1915 through 1925. Zapt is an eccentric inventor whose creations, needless to say, never go quite right. His "ammunition exploder," designed to stop the war in Europe, sets off the wrong explosives; his antigravity paste gets blown out the window and levitates the wrong people; his invisibility liquid ends up being hosed on Zapt's house and everyone in it; and his supergrowth food is fed to a flea, which grows to giant size.

Dr. Zeng Tse Lin. The good Doctor was created by Walt Bruce and appeared in Popular Detective in the early- and mid-1940s. "Dr. Zeng" was actually Robert Charles Lang, a white American who was born to missionaries in China. He was raised in China and had much appreciation and respect for the Chinese and their culture and history--a surprising amount for a series of stories written in the 1940s. On reaching adulthood he became a doctor and then, with the advent of the war, left China to return to America. He set up shop in San Francisco's Chinatown, pretending to be the Chinese "Doctor Zeng Tse Lin" so as to gain the trust of both the Chinese-Americans and the Anglo-Americans in San Francisco. In this guise he is both a doctor and a crime-fighting detective.

Zenigata Heichi. Zenigata Heichi was created by Nomura Kodo and appeared in 338 stories and novels, beginning with "A Virgin in Gold" in 1931. Zenigata was created not only as another Sherlock Holmes homage but also an homage and rival to Hanshichi. Zenigata is active during the Edo period (Japan from 1601-1867). Zenigata has certain similarities to Holmes and Hanshichi, but is distinctive in other ways; he is poor, and although he is a Goyoukiki (the private assistants to the Doshin, or regular police officers during the Tokugawa shogunate of the Edo period) he takes no money from those he helps. He is sometimes called "Shikujiri Heiji," or "Heiji the Failure," because he occasionally lets criminals go. This is not due to incompetence on his part but rather to the fact that, like Holmes, he has compassion for some criminals--he hates crime but not the criminals themselves. Heiji has a great deal of sympathy for the common man and dislikes samurai and men of power, despite his position in the government. He drinks a little and smokes a lot, he has no taste for fame and is perfectly content when others take credit for his accomplishments, and he is particularly skilled at using heavy coins as a throwing weapon. He is Watsoned by Hachigoro, a tall, easy-going man who is not only Zenigata's assistant but also a veritable newspaper of Edo, functioning as a mouthpiece for Nomura to tell Zenigata and the reader all about current events in the capitol. Zenigata is also married to Oshizu, who is Zenigata's Mrs. Hudson.

Zenith the Albino. See The Zenith the Albino Page.

Zero the Silent. Zero the Silent, creator unknown, appeared in Adventure in 1931 and 1932. He was a disgraced policeman who used a scientifically-advanced skin-tight costume (note that this get-up predated all the superhero costumes which appeared later in the decade) which had suckers on the hands and knees, allowing him to crawl, Spider-Man-like, along walls and ceilings. Attached to his wrist was a "throw ball" on an elastic; Zero bowled googlies at his opponents, with the elastic bringing the ball back to him.

Zigomar (I). See the French Heroes section.

Zigomar (II). Zigomar (II) was created by Nikola Navojev and Branko Vidic and appeared in Mikijevo carstvo, a Serbian magazine appearing in 1939. Zigomar (II) was a masked avenger type not dissimilar to the Phantom; in the words of one critic, Zigomar (II) also wore

a costume clinging tightly to his body, and a mask on his face; on his hand was an engraved ring with an engraved letter Z. But unlike Phantom whose inseparable companion was the tamed wolf Devil, this hero, Zigomar, adorned additionally by a black cape, had a different companion, a short Chinese named Chi Yang.

Dr. Zimmertür. Dr. Zimmertur was created by the Swedish writer Frank Heller, the creator of Filip Collin, and appeared in five books, the first being Doktor Z (1926). Dr. Zimmertur is a Swedish psychoanalyst  and crime-solver, an eccentric Jewish gentleman whose methods were taught to him by Mycroft Holmes himself.

Zolok. Zolok was created by Harry Revier and appeared in the twelve-chapter serial The Lost City (1935). (Zolok is opposed by a Thud Broncchest type named Bruce Gordon, but he’s so thunderingly dull that he doesn’t deserve an entry to himself) Zolok is a mad scientist who intends to rule the world from his underground city (the metropolis of the film’s title). He plans to carry this out with the help of his technologically-advanced weapons, which include a weather-control machine, a “rejuvenation ray,” a zombie-maker, a death-ray, and (more disturbingly) a machine for turning those of African descent into pale honkies, and vice-versa.

Zoobilly Boys. No, really. The Zoobilly Boys. They were created by "Ima Kidder" and "Hugh Binhad," which makes me think that the reference work I'm taking this from might be taking the piss. They appeared in the five book "Zoobilly Boys" series, which began in 1923 with The Zoobilly Boys in Tierra del Fuego or Captured by Penguins on the Ice. The Zoobilly Boys were two boys who grew up over the course of their adventures and became a policeman and an engineer, all the while exploring Tierra Del Fuego, finding the "phantom iguana," "yaking it up with the Dali Lama" in Tibet, "egg rolling on Monday" on Easter Island, and discovering a Greek Atlantis.

Zorka, Alex. Dr. Alex Zorka was created by Wyllis Cooper and appeared in the twelve-part serial The Phantom Creeps (directed by Ford Beebe) (Ford Beebe! Ford Beebe! Ford Beebe!) (1939). Zorka is a mad scientist looking to take over the world. He gets taken down by plucky newspaperwoman Jean Drew and Plunk Beefjaw military officer Captain Bob West, but not before giving them a run for their money with his “devisualizer belt” (which makes him invisible), killer robot, suspended animation gas, and mechanical explosive “death spiders.”

Zorro. You are all, I trust, familiar with the story of Zorro? Don Diego Vega, that contemptible Spanish fop who turns into the justice-seeking masked Mexican mystery man Zorro at night? Zorro, created by Johnston McCulley, appearing in All-Story-Weekly and Argosy from 1919 through 1951? That Zorro? Yes, I'm sure you are familiar with him. Which is why I don't need to give you any more information on him, but instead can send you to this site, which has lots of lovely information on him:

The Official Home of Zorro!
Rather an entertaining site, I think.
 

Introduction
A. The Abbey Girls to Dusty Ayres
B. Bagley to Scott Burton
C. Orhan Cakiroglu to Dr. Theodore Cunliffe
D-E. Dana Girls to Don Everhard
F. Ralph Fairbanks to Miss Fury
G. The Gadget Man to G-8
H-I. Dr. Hackensaw to Baron Ixell
J. Jack, Doc & Reggie to Justice Syndicate
K. Calvin Kane to Kwa of the Jungle
L. Major John T. Lacy to Langhorne Lyte
M. Professor Maboul to Mr. Mystic
N. Lee Nace to Nyoka
O. Fergus O'Breen to Ozar the Aztec
P.  Penny Packer to Judge Pursuivant
Q.  Oliver Quade to Sebastian Quin
R. Ed Race to Captain Rybnikov
S. The Safety First Club to Tom Swift
T-U. Tahara to Godfrey Usher
V. Lieutenant Valcour to Norton Vyse
W. Inspector Wade to Dr. Xavier Wycherley
X-Z. X Bar X Boys to Zorro
Links.