In the late 1940s after World War II, it was discovered that a huge portion of comics readers were young men in the armed services. Marvel Comics started to shift their content to appeal to the GI readership, with lots of pin up beautiful women front and center. Lots of heroes got female sidekicks or female counterparts, like the Human Torch’s Sun Girl, Captain America’s Golden Girl, Namor’s Namora, as well as characters like the Blonde Phantom.
Since most of the characters in this era were introduced after World War II, Marvel almost never revisits them, because most of their flashback stories are set during the war (Sun Girl has not had a single Marvel appearance since 1949). And you almost never see this era discussed in books about the history of Marvel, where it is typically overlooked.
Art by Bill Ward, Alex Schomburg, Bill Horn, among others.