Atomicus' origin was unknown. However, several theories were made:
- He is an alien intelligence was trapped in an atomic reactor. Then, it somehow manifest a human-like form.
- He is an ancient entity reborned into modern day as an atomic-themed superhero.
- He was an air force lieutenant reborn after being killed via atomic powers.
His code-name came from a newspaper calling him "Atomicus".
Atomicus' greatest weakness was an almost-childlike naivete about human nature accompanied by a sad frustration with the ubiquity of human irrationality, a weakness with which he coped by creating a secret identity as Adam Peterson. Atomicus could disguise this weakness through the persona of a lovable loser when masquerading as Adam Peterson, so he used this alias as a means to obtain a safe space for recovering psychologically from the constant and often unfair demands of the world. Despite his social awkwardness and naivete, he was able to pair up with a girlfriend, the intelligent and sophisticated Irene Merriweather.
Irene Merriwether constantly tried to prove Atomicus and lovable loser Adam Peterson were the same person, but Atomicus always outsmarted her efforts. Unfortunately for the couple, Irene misinterpretted this as a flirtatious battle of wits, thinking that Atomicus was waitingly playfully for her to uncover his secret identity, whereas Atomicus actually found the whole thing bewildering. Eventually, Irene broke down and tore off Adam's jacket, revealing his Atomicus costume publically and destroying his alias as Adam Peterson in the process. Feeling betrayed and unable to understand why Irene would do this to him, Atomicus left Astro City and likely the entire planet Earth since he hasn't been seen since, to Irene's sorrow and guilt.