Comic book superhero, published by DC Comics. The character was created by writer Geoff Johns for the "Stars and S.T.R.I.P.E." series in 1999.
Stargirl was actually Courtney Whitmore, the teenaged stepdaughter of Pat Dugan, who had secretly been known during DC's Golden Age as Stripesy, the adult sidekick of the teenaged hero, the Star-Spangled Kid. Courtney didn't like Pat and hated having to move from Los Angeles to boring Blue Valley, Nebraska. When she discovered Pat's secret identity while unpacking moving boxes, she also found the original Star-Spangled Kid's Cosmic Converter Belt; wearing it gave her some measure of enhanced strength, speed, and agility, as well as giving her the ability to fire "shooting stars" at enemies. Hoping to spite Pat and, she hoped, convince her mother to divorce him, she became the new Star-Spangled Kid, fighting the forces of evil in Blue Valley. Pat, meanwhile, built an oversized suit of armor called S.T.R.I.P.E. so he could keep an eye on his stepdaughter and keep her out of serious trouble.
Eventually, Courtney reconciled with Pat, and she started calling herself Stargirl, after Jack Knight, the retiring Starman, presented her with the Cosmic Staff that the original Starman invented. Now able to use the Cosmic Staff to fly and manipulate cosmic energy, she joined the revitalized Justice Society of America, quickly becoming one of the comic's most popular members. While a member of the JSA, Stargirl has fought supervillains, traveled through time, been temporarily aged to adulthood, and served as a mentor for other young members of the Justice Society.
Stargirl learned that her biological father worked as a supervillain minion and eventually came to terms with his preferred line of work after he died in a storm. She briefly dated Captain Marvel -- his alter ego, Billy Batson, is actually a year or two younger than Courtney -- but he broke it off because people who weren't aware of his secret identity would think he was robbing the cradle. She's also been close to Atom Smasher, a character who has frequently crossed over from hero to villain and back again.
Geoff Johns has said that he based Stargirl's personality on that of his sister, Courtney Johns, who died in 1996 in the explosion of TWA Flight 800. With any luck, her status as an almost-real person should make her immune from DC's all-too-frequent fits of "Let's kill some random chick just so everyone knows we're badass."