Rhonda Pineda was created by writer Geoff Johns, with her first making an appearance in DC's The New 52 FCBD Special Edition in 2012, though not properly introducing her until the next year in Justice League #18.
Major Story Arcs
Joining the Justice League
Rhonda Pineda began to assist the Justice League during Throne of Atlantis, when the Atlanteans invaded the surface world, lead by Aquaman's brother Ocean Master. Rhonda was one of the many heroes to answer the call for assistance, though no one noticed her due to her size. She played a key roll in disarming the bombs that would have sunk Boston into the Atlantic Ocean. She came to the League's attention and received an invitation to their satellite to join other would-be candidates for an expanded League. She made her power known after assisting in disabling a malfunctioning Platinum and capturing her value resonmeter. She was a student at Ivy University.
It was later revealed after a battle with Despero that she had been an agent for Amanda Waller the whole time. Waller intended her to be the JLA's counter to Element Woman . She infiltrated the team (again, through Waller's interference) to spy on her new teammates Firestorm and Element Woman with the mission to discover how to render their respective powers inoperable in battle. She was one of the only people aware of an impending war.
It was finally revealed that that Rhonda is actually from the Earth 3, the home of the Crime Syndicate of America. She worked with the The Outsider to frame Superman for the murder of Arthur Light by going into Superman's brain and using a sliver of Kryptonite from Batman's ring to hit a nerve and trigger his heat vision. This is what was subsequently weakening Superman throughout the arc. It was revealed that she told Cyborg that she was the spy on the team and mentioning that she was the one who went into Superman's ear drum, into his brain, to set off the heat vision that killed Dr. Light. Cyborg was going to tell the team, but, before he could, his cybernetic body, badly damaged by Atomica, split from his remaining biological elements and formed the super villain named Grid. The Justice League and Justice League of America were in shock and fell quickly to Atomica's true allies the Crime Syndicate from Earth 3.
After the events of Trinity War, the Crime Syndicate begin wreaking havoc upon the world. Without the three Justice League teams to fight against them, other hero teams are forced to step up. The Teen Titans decide to directly engage in combat with Johnny Quick and Atomica but are quickly defeated and sent into the future by Johnny Quick. The Doom Patrol is another team that decides to step up, however, they too are defeated when Johnny Quick and Atomica seemingly kill of Scorch, Karma, and Negative Woman.
During the events of Forever Evil: Rogues Rebellion, Atomica and Johnny Quick are sent after the Rogues. When they catch up to them, the Rogues are already fighting The Royal Flush Gang, but this doesn't stop Atomica and Quick from stepping in and taking control. When some cops show up Atomica jumps into one of their heads and grows making his head explode. When Weather Wizard concentrates on stopping Quick Atomica shrinks and hides. Before Quick can defeat Wizard him and Atomica are called back to the Syndicate's headquarters.
When Luthor's Injustice League team along with Batman infiltrate the fallen Watchtower, Captain Cold freezes and smashes one of Johnny Quick's legs. Cold and Black Manta also release the prisoner the Crime Syndicate was holding, who is revealed to be Alexander Luthor. Alexander then calls upon the power of Mazahs and a lightning bolt strikes the compound, Alexander then proceeds to kill Johnny Quick by snapping his neck and stealing his powers. Atomica is knocked out under a pile of rubble caused by the explosion and wakes up after the battle is over. She appears shocked and states that her size changing belt has been shorted out and she is stuck in a smaller form, asks where Johnny Quick is but Lex Luthor tells her that he is dead and proceeds to squash her with his foot, and thereby killing her.
Originally she was described by Cyborg and other members of the League as a shy, but heroic, young girl. She has been shown to use her powers for the purpose of going into video games.
She pretended to show regret for her loyalty to Amanda Waller, even offering to give her suit up to "atone" for the things she did at Waller's behest. It was eventually revealed to be a complicated ruse due to her deceptive, and arguably evil, nature.
Although she initially seemed like a kind, loyal member of the League, it had all been an act. She was actually a cruel and deceitful member of the Crime Syndicate of America.