Like most comics of the time, Pep initially revolved around the type of superheroes that flourished in comics' Golden Age. In fact, it was Pep that introduced the first of the patriotically-themed superheroes in the form of the Shield, who pre-dated the ultimate patriotic-superhero, Captain America, by over a year. The Shield arrived in the first issue, rendered by artist Irving Novick. Also appearing were the Comet (by Jack Cole), the Rocket, and a number of other minor adventure characters. MLJ's superhero lineup was not necessarily any better or worse than any other publisher's; various comics packagers were pumping out this new superhero thing by the dozens, especially after the beginning of World War II. Notably, Pep did feature the first death of a superhero - the Comet was killed in #17.
Although it probably didn't seem very significant at the time, issue #22 launched a new humor strip, based loosely on Mickey Rooney's Andy Hardy film character. This was Archie, the red-haired girl-crazy teenager whose four-color exploits would come to define the world of the modern teen for several decades (or, at least, an idealized world). Archie didn't appear on the cover of that issue, and in fact he wouldn't appear on a cover until #36, held aloft by the superheroes the Shield and the Hangman; he made the cover again with #41 several months later - after which he all but took over the cover (and the book) completely. The Shield would continue to appear in the book until #47, but no longer on the cover; that spot was now reserved for Archie and his gang.
Archie's teenaged antics took America's comics readers by storm - at least the girls, and probably several of the boys as well. MLJ was smart enough to sniff which way the wind was blowing, because by the end of World War II, superheroes were out, with only a very small number (including Batman, Superman, and Wonder Woman) surviving out of what once were hundreds. MLJ started banking on their successful humor strips, centered on the Archie character, so much by that by early 1946 the company's very name had been changed to Archie Publications.
Pep continued to be a vehicle for the Archie gang's adventures until its eventual demise. The title, being a general anthology, featured nearly all of the line's character's at one time or another, and had even introduced most of the important ones. Jughead and Betty had been introduced at the same time as Archie, in #22; Archie's iconic jalopy would arrive with #25; Veronica completed the classic love triangle with #26; and they would soon be followed by Mr. Lodge, Miss Grundy, Mr. Weatherbee, Li'l Jinx, and more.
For nearly 50 years Pep reigned as the premier Archie comic, and one of the few Archie-gang books that didn't have a character's name in the title.