I'm a bad little girl!
Atomica was initially believed to be a member of the Justice League, but she later admitted to being a double-agent for the Justice League of America in order to keep an eye on the world's most powerful heroes. Even that was a lie, as she was actually a denizen of Earth-3 and working to gather all of the Justice Leagues into one place to weaken for the arrival Crime Syndicate.
History
Atomica was the girlfriend to Johnny Allen. The two of them were Bonnie and Clyde-type outlaws who were on the run from the police (made up of the Earth-3 Rogues). They stumbled onto a laboratory and gained superpowers, with Johny gaining super-speed and Atomica acquires a device that gives her the ability to shrink. They later team up with the Crime Syndicate.
Atomica slipped a shard of kryptonite into Superman's brain in order to keep him down while the Syndicate invaded Earth. She and Johnny terrorised the remaining superheroes and any criminals who had a change of heart. Eventually the Injustice League fight back, with Black Manta going up against Atomica. Atomica tries shrinking to a size small enough to slip into Balck Manta's suit, but finds that it's sealed at an atomic level. When Alexander Luthor is released, Atomica's shrinking device is damaged.
In the aftermath, Lex Luthor finds Atomica and squashes her.