Thunder was created by Jerry Ordway and Mike Manley.
Her first appearance was in 1996.
Thunder is the super-powered alter-ego of a young girl from the planet Binderaan, circa 9,000 A.D.
The power of Shazam is bestowed upon her by Captain Marvel himself.
After a mission to the 20th century, Thunder is sent to the 30th century, when pro-science attack and destroy the Rock of Eternity, a major source of all magic. After subduing some of the terrorists with the help of member of the Legion of Super Heroes she is offered membership. which accepts while continuing her mission to collect all the scattered pieces of the Rock of Eternity.
Instead of saying "Shazam" to invoke her powers she says "Captain Marvel", that's a neat little spin.
I've actually worked with Mike Manley at Warner Bros a while back on some cartoons Bruce Timm was producing. Good times!
Slowly but surely more Legionnaires are coming your way...