Comic book superhero, originally published by Fawcett Comics and currently owned by DC Comics. She was created by Otto Binder and Marc Swayze and made her debut appearance in "Captain Marvel Adventures #18" in 1942.
Mary's full name is Mary Batson -- or Mary Bromfield, as she is adopted. She is the twin sister of Billy Batson, better known to the world as the high-flying superhero Captain Marvel. In the original Fawcett comics, the Batsons' parents died in a car accident. When told she had to send both children to an orphanage, their nurse decided she wanted to get at least one of the children into a proper home. A baby girl had very recently died in the nurse's care, so she managed to substitute Mary for the deceased baby. So while Billy was stuck at the orphanage for years, Mary grew up as the daughter of the wealthy Mrs. Bromfield.
Years later, after Billy has become, of all things, a teenaged announcer for the WHIZ radio station, he hears from the nurse, who is dying and wants to tell Billy how to find his sister. She gives Billy a locket that's been broken in half and tells him his sister wears the other half. Billy remembers that one of the contestants at a radio quiz he hosted recently was a rich girl named Mary Bromfield who wore a broken locket. He and his friend Freddy Freeman (in their superheroic identities as Captain Marvel and Captain Marvel Jr.) track Mary down and end up rescuing her from a band of kidnappers. After discovering the two halves of the lockets match, Billy and Freddy reveal their secret identities to Mary, who asks if she could get superpowers herself. Billy scoffs that the wizard Shazam would never give powers to a mere girl! Because it's the Golden Age, however, Mary doesn't react by punting Billy's junk into his ribcage.
Of course, the kidnappers take that moment to regain consciousness and attack Billy and Freddy, gagging them so they can't say their magic words. And when Mary says "Shazam!" she gets struck by the magic lightning and given powers like Captain Marvel! (Although Billy was transformed into an adult as Captain Marvel, Mary still had the appearance of a teenager.) She beats up the bad guys, and then the wizard appears and reveals that instead of getting powers from Solomon, Hercules, Atlas, Zeus, Achilles, and Mercury (like Captain Marvel), her abilities came from Selena, Hippolyta, Ariadne (later changed to Artemis), Zephyrus, Aurora (later Aphrodite), and Minerva. Mary continued to appear in the various Captain Marvel comics, writing up their adventures in her Ledger of Good Deeds.
The Marvel Family went on hiatus for many years beginning in the early 1950s, after National Comics (the owners of Superman) sued Fawcett, claiming Captain Marvel was a shameless copy of Superman. They got a judge to side with them, and Fawcett went out of business. (The legal term for this is fuckin' buuuuullshit.) Years later, DC Comics acquired the rights to use the Marvels, and gave them all updated and much more complicated origins to fit into their new universe.
In a 1994 graphic novel called "The Power of Shazam!" by Jerry Ordway, the Batsons took Mary along on a trip to Egypt but left Billy at home in America. The Batson parents were killed by Teth Adam (a.k.a. Black Adam), whose sister arranges for the Bromfields, her childless but wealthy employers to adopt her illegally. Meanwhile, Billy was homeless, but had already encountered the wizard Shazam, who'd given him his powers. He learned that Mary was still alive, but hadn't been able to find her after several years of searching. To make a long story short, he finally tracks her down with the help of a magical "Tawky Tawny" doll, which then gets her to say "Shazam" when Billy gets tied up and gagged. In the updated continuity, Mary is empowered by the same beings who empower Billy and Freddy, and instead of apearing as a teenager, superpowered Mary takes on the appearance of an adult. Sometimes she's even been referred to as "Captain Marvel" -- or "the female Captain Marvel," to differentiate her from Billy. But "Mary Marvel" is a perfectly good name, so it gets used as often as any other name.
The Marvels only rarely had their own comics, so Mary didn't get to appear very often. Perhaps her best moments came in the "Formerly Known as the Justice League" and "I Can't Believe It's Not the Justice League" miniseries by Keith Giffen, J.M. DeMatteis, and Kevin Maguire. In this comedic series, in which Mary is recruited into a low-rent superteam called the Super Buddies by former Justice League financier Maxwell Lord, Mary is depicted as very innocent and naive. At one point, the team is abducted by Roulette, a supervillain who runs a secret gladiatorial arena for metahumans. Mary and Captain Atom are assigned to battle each other. Of course, they're not interested in beating each other up so a bunch of bad guys can get their jollies. Unfortunately, the bad guys activate an insta-hypnosis effect to make them want to kill each other -- and Mary, sporting a dementedly glorious ax-crazy grin, almost beats Captain Atom to death, as well as severely injuring the heroine Fire when she tries to intervene. Once the hypno-beam is switched off, the Super Buddies have several more adventures against aliens and demons and mirror-universe doppelgangers before the series was ended.
And after that, things pretty much went to shit for Mary for a good long while.
There are few things DC Comics hates more than a character that isn't grimdark. And the Marvels had been soaked in Golden Age innocence, humor, and whimsy from their very beginnings. That couldn't be allowed to stand!
In the "Day of Vengeance" miniseries in 2006, the Spectre decides to destroy all traces of magic in the universe and succeeds in killing the wizard Shazam in the final issue of the series. As a result, the Marvels are all deprived of their powers -- in Mary's case, while she was flying three miles in the air. She manages to survive -- barely -- but feels lost without her powers. During the "Countdown" weekly series in 2007, she receives the powers of Black Adam -- giving her a black costume instead of the white costume she'd had for years. She ends up getting corrupted by these powers, giving them up, getting powers from Darkseid, getting possessed by sadistic New God torturer DeSaad, getting an even more ridiculous costume involving a mostly shaved head and dyed purple hair, corrupting Billy into a supervillain, getting her powers taken away again by a reborn Shazam...
...and sadly, that was basically it. She's reappeared briefly in the New 52 and Rebirth continuity reboots of DC but always as a minor character and never with any permanent powers.
She's fared significantly better in non-continuity comics. In 2007, Jeff Smith made a very traditional and light-hearted Captain Marvel miniseries with "Shazam! The Monster Society of Evil," in which Mary shows up as the young sister of Billy. She acquires powers similar to Captain Marvel's, but she remains a young pre-teenager. The followup series, "Billy Batson and the Magic of Shazam!," by Mike Kunkel, Art Baltazar, and Franco Aureliani, saw Mary briefly gaining an adult form while fighting Black Adam. In Grant Morrison's world-hopping limited series "The Multiversity," a full issue is devoted to the sunshine glories of the Marvel Family in 2014's "Thunderworld." The same held true in the similarly dimension-crossing "Convergence" series in 2015, in a remarkably upbeat issue created by Jeff Parker and Evan Shaner.
The positivity, humor, and retro feel of these non-continuity Marvel Family comics always get high marks from readers and reviewers, but DC seems intent on turning the Marvels into angsty edgelords. But maybe there's future hope for Mary and the rest of her family...