Rowan Hansen, from Illinois, wrote "I love your comics, but I would love them a lot more if there were more girls.
"There are Superman and Batman movies, but not a Wonder Woman one. You have a Flash TV show but not a Wonder Woman one. Marvel comics made a movie about a talking tree and raccoon awesome, but you haven't made a movie with Wonder Woman. Please do something about this. Girls read comics, too, and they care."
"I have never really understood why they've had more male superheroes," Rowan told USA Today.
"If Batman gets to wear armor, then why doesn't Wonder Woman get to wear armor? And I know that she's kind of invulnerable, but it would still be nicer if she didn't wear a bathing suit all the time."
Rowan's father Jim posted a picture of the letter online and it soon went viral.
By 30 January, DC had responded on Twitter to assure her they were working on bringing a stronger female cast to the DC Universe.
After a short while, the folks from DC presented Rowan with a sketch of herself as a superhero.
Moved by the gesture, Jim Hansen said he would be displaying the picture on their mantle for all the see.