Appearance: Two-Gun Kid #1 (March 1948).
Appearances. All-Western Winners #2-4, Best Western #58-59, Black Rider #19-23, Blaze Carson #4, Gunsmoke Western #57, 59, 60-63, Kid Colt #1, 125, Mighty Marvel Western #1-46, Rawhide Kid #40, 66, Two-Gun Kid #1-136, Western Winners #5-6, Wild West #1-2, Wild Western #3-6, 9, 11, 12, 33-39, 41, Two-Gun Kid: Sunset Riders #1-2, Blaze of Glory #1-4, Black Panther v2 #46.
Years Active: 1870s-1885.
Originally, during the Golden Age, the Two-Gun Kid was Clay Harder, a singing cowboy who wandered the Old West dispensing justice. He did not wear a mask.
Later, during the Silver Age relaunch of the Two-Gun Kid, it was revealed that the first 59 issues of Two-Gun Kid, and presumably every pre-Silver Age appearance of the Two-Gun Kid, were dime novel stories within the Marvel Universe; that is, they had never happened in Marvel continuity. The Two-Gun Kid was now Matthew Liebowicz, born in Boston, a Harvard-educated lawyer who settled in Tombstone, Texas, in the 1870s. Soon after arriving in Tombstone he was attacked by a gang of roughnecks. Soon after that Liebowicz attempted to help an old man who was being harassed by the same gang that had attacked Liebowicz; the old man, the legendary gunfighter Ben Dancer, scared the gang off by drawing his gun, and then befriended Liebowicz and taught him how to shoot. Liebowicz became Dancer's superior as a shooter and eventually as a cowboy. Liebowicz took Dancer's advice and kept his skills secret, becoming the costumed Two-Gun Kid (Liebowicz was inspired to take the name by the dime novels about Clay Harder) and working as a lawyer under the name of Matthew J. Hawk.
As the Two-Gun Kid, Liebowicz/Hawk, armed only with his trusty six-shooters and his horse Cyclone, had a wide variety of adventures. He fought costumed desperados, like the Rattler and the Hurricane, the latter of whom had superspeed. Two-Gun encountered a Kirby Monstertm in "The Monster of Hidden Valley." He teamed up with various other heroic cowboys, such as the Rawhide Kid and the Ringo Kid. He even aided the modern heroes the Avengers when they traveled back in time to the 1870s. In 1873 the Two-Gun Kid accompanied the Avengers forward in time, to the modern era, after another Avengers adventure in the past. Two-Gun spent a brief period in the 20th century before returning home.
In 1883 or 1884 Two-Gun got involved with the Sunset Riders. With them he faced off against the agents of several governments, including the American and British. During that series it was revealed that Two-Gun's wife Nancy died of a ruptured cervix while giving birth to their daughter, Nancy, who died of rubella at age 4. The series also revealed that Two-Gun's longtime pal Boom-Boom Brown died of gangrene from bad dentistry about six months before the miniseries. (The Old West was a rough place.) At the end of that series Matt Liebowicz/Hawk, and the Two-Gun Kid, are both killed. Or so The Sunset Riders showed.
Blaze of Glory retconned that away, thankfully, and instead stated that it the deaths were a ruse by Liebowicz/Hawk. By 1885 Two-Gun had settled into the boom town of Anaconda, Montana, and was working as a lawyer under the name of "Clay Harder." In Blaze of Glory Two-Gun goes to help Reno Jones and the town of Wonderment against an attack by Kid Cassidy and the Night Riders. Two-Gun has one final team-up with the Rawhide Kid, Kid Colt, and the Outlaw Kid, but Two-Gun dies while defeating the Night Riders.
Notes: I think dismissing the first sixty issues of Two-Gun Kid as "fictional" was a big mistake, exactly the sort of retcon that DC is (rightfully) assailed for by self-righteous Marvel fans. Two-Gun Kid #1 was the debut of the Western genre at Timely/Atlas, along with Wild West #1. The Two-Gun Kid is the symbolic father of all the rest of Marvel's Western heroes. Clay Harder deserved better than to be waved away as a dime novel character.
The image at the top is Two-Gun as he originally appeared; the image below that is the revised, Silver Age Retconned Two-Gun.