For most of their history up until about 1959, DC's war and science fiction magazines had not featured continuing characters. Instead the stories had been one-shots, with characters never returning. Apparently a decision was reached by management to require continuing features, and very quickly they were put in place:
March 1959: All American Men of War #67 introduced Gunner and Sarge. The team continued in the next issue, before moving over to Our Fighting Forces, beginning with #45, the May 1959 issue.
April 1959: Sgt Rock made his first appearance in Our Army at War #81 (as Sgt Rocky).
May 1959. Tank Killer makes his first appearance in All American Men of War #69.
May 1959. The Space Museum opens its doors for the first time in Strange Adventures #104. It would eventually settle into a rotation with Star Hawkins and the Atomic Knights, with each feature appearing every third issue, until Julius Schwartz turned the editorship reins over to Jack Schiff in 1964.
August 1959: Mademoiselle Marie, a French resistance fighter, debuted in Star Spangled War Stories #84.
August 1959. Mystery in Space, which had occasionally had regular features but not for awhile at the time, began running the Adam Strange series, starting with #53.
August 1959. Space Ranger became the regular feature in Tales of the Unexpected #40.
August 1959. House of Secrets #23 introduced Mark Merlin, a supernatural detective/debunker.
March 1960. Star Hawkins begins in Strange Adventures #114.
April-May, 1960. Star Spangled War Stories publishes the first of many zany War that Time Forgot stories, a series which featured US soldiers battling dinosaurs and other oddball creatures (including a memorable white King Kong).
June 1960. The Atomic Knights make their first appearance in Strange Adventures #117.
Nov-Dec 1960. Johnny Cloud, Navajo ace pilot of World War II debuts.
May 1961. GI Combat #87 hosts the first story featuring the Haunted Tank.
It should be noted that several titles held out even longer; My Greatest Adventure and House of Mystery did not have recurring features until May 1963 (Doom Patrol, #80) and June 1964 (Martian Manhunter, #143). And of course DC's romance titles resisted continuing characters until Heart Throbs and Secret Hearts began running their soap opera series around 1966.