Ryan Choi first appeared in DCU: Brave New World and was created by Gail Simone and Grant Morrison. Ryan Choi, as described by DC solicitations, is "a young hotshot professor who's filling the extra spot on Ivy University's teaching staff. .. and who inadvertently ends up filling the old Atom's super-heroic shoes". This new Atom is based on a redesign by Grant Morrison.
Choi makes his first appearance in the new Rebirth continuity in the Justice League of America: The Atom one shot, by Steve Orlando and Andrew MacDonald. His suit is redesigned to resemble Ray Palmer's in the Arrowverse TV shows.
Born in Hong Kong, Ryan Choi was a longtime protégé of Ray Palmer who had been corresponding with him through letters. After Palmer's disappearance, Ryan moved to Ivy Town in America to assume his mentor's place on the staff of Ivy University. Following clues left by Palmer, Ryan discovered a "bio-belt," allegedly the size and density-manipulating device used by his predecessor, and became the new Atom with Palmer's apparent blessing. Though taken with the superhero lifestyle, Ryan is a scientist first and foremost and approaches many of his adventures from the perspective of scientific discovery and investigation.
Since taking his mentor's place, Ryan has found himself at the center of a conflict between the forces of science and magic. It has been claimed that the impossible feats performed by Ray Palmer during his superheroic career caused the very fabric of reality to warp in Ivy Town's vicinity, making it a nexus of paranormal activity. Many parties, including the ancient "Cancer God" M'Nagalah and the microscopic aliens known as "The Waiting," consider Ryan a key player in the war and have made attempts to recruit, capture, or kill him. He is advised by among others Ivy Town Police Chief, Liza Warner (a.k.a. Lady Cop).
As The Atom, Ryan has faced numerous challenges, including the shrinking serial killer Dwarfstar, his strict and disapproving father, and being seduced, kidnapped, and even swallowed alive by the size-changing villainess, Giganta. Through it all, his ingenuity and keen deductive mind have served him in good stead.
Ryan Choi was involved in the search for the missing Ray Palmer, traveling into the restored Multiverse along with Donna Troy, Jason Todd and a Monitor nicknamed "Bob". Literally plucked back to New Earth, he leaves his role of dimension-hopper to Kyle Rayner, returning to defend Ivy Town from a monster invasion. Later he is led to a mistaken belief that Ray Palmer has become an egocentric madman, and Ryan himself may be only a pawn of his mad fantasies. This is later revealed to be a ploy by Ray's old nemesis, Chronos. The All New Atom series ended with issue 25, when Ryan, with some help from the returned Ray Palmer, is able to discern between the truth and the lies fed by Chronos and his new assistant, Lady Chronos, a former sweetheart of Ryan turned to crime. Ryan eventually discovers that Ray Palmer never knew of Choi: instead the bio-belt was a tainted gift from Jia, and the Ray Palmer letters a clever forging by Chronos, meant to force Ryan into accepting the Atom mantle, and taking the blame for the staging menaces sent against the city. However, due to Ryan's ability into sorting out the mess, besting the Chronos couple and restoring Ivy to normalcy, Ray finally gives him his blessing.
Ryan expresses his desire to find a new identity for himself, since Ray, despite giving him his blessing earlier, had resumed using regularly his Atom identity. In Justice League: Cry For Justice #1, Ray and Ryan are seen fighting Killer Moth together, and at the end of the battle both of them show respect towards each other, with Ray asking Ryan to continue using the Atom name.
During the Brightest Day event, Ryan is murdered by Deathstroke and his new team of Titans during their first mission. His corpse is then delivered in a matchbox to Dwarfstar, who is revealed to be the person who hired the Titans. His death became the subject of racial controversy, as Ryan had been one of the few high-profile Asian characters in the DC universe and was appearing on television at time of his death. A short time after Ryan's death, Deathstroke is briefly shown dismantling his bio-belt for some unknown purpose. In an interview done during Comic-Con International 2010, Titans writer Eric Wallace stated that Choi's death would have major repercussions for the team, and would bring the Titans into conflict with the wider DCU.
Later, Ray begins an investigation into the disappearance of Ryan who, unbeknownst to the superhero community, has been murdered. Ray comforts Ryan's girlfriend Amanda, and muses that Ryan may be hiding out like Ray did after the events of Identity Crisis. Amanda Waller eventually tells Giganta about Dwarfstar's hand in Ryan's murder, though it is unknown if she revealed the involvement of Deathstroke and the Titans. After stealing Dwarfstar's belt (thus rendering him powerless), Giganta pummels him into submission and tapes his mouth shut, telling him that she plans on taking her time to torture him. Later, Ray discovers evidence that Dwarfstar had a hand in Ryan's death, and vows to find him and make him pay. Ray eventually finds Dwarfstar in a hospital, where he is recovering from the severe injuries he sustained from his torture at the hands of Giganta. Believing that it may lead to a lighter sentence, Dwarfstar confesses to hiring Slade to kill Ryan. Armed with this knowledge, Ray leaves to inform the Justice League, but not before telling Dwarfstar that Deathstroke will likely kill him for his betrayal. The members of the Justice League finally confront Deathstroke and Titans on their way back from a disastrous mission, intending to arrest them for Ryan's murder. Ray seriously injures Deathstroke for killing his friend, but the Titans ultimately escape due to the intervention of Isis and Osiris. After failing, Ray sets out to write the eulogy for Ryan's funeral, with encouragement from Superman. It is also revealed that Deathstroke dismantled Ryan's bio-belt to utilize the technology to revive his dying son, Jericho. Later, Ray, Amanda, the Justice League, the Teen Titans and numerous other heroes are shown at the funeral honoring Ryan's memory.
At San Diego Comic-Con 2011, artist Jim Lee revealed that Ryan would be one of the members of the new Justice League title drawn by Lee and written by Geoff Johns. The undoing of Choi's death will be one of the numerous changes to DC's continuity caused by the Flashpoint event.
During the first story arc of the series, it is mentioned in passing that as a young graduate student, Ryan had helped design one of the components of Cyborg's robotic body.
In the Convergence crossover, when the alternate Brainiac miniaturized the universe of the New Earth, Ryan had appears to be alive and confronts Ray Palmer was battle Angor universe's Barracuda. Ryan reveals that after his death, his consciousness had survived in the universe where the Atoms' masses are shifted to whenever they change size. He later returns to the realm of the living after appropriating the flesh from Ray's severed hand to create a new body for himself. After Barracuda is defeated, the two Atoms work together to defeat Deathstroke, avenging Ryan's murder.
Ryan makes his official debut in the new Rebirth continuity as a teenage genius Ivy League college student tutored by Ray Palmer. Palmer reveals his identity as The Atom and enlists Ryan's help in fighting crime, talking to Ray from his lab in a tech support role. One day, after many adventures together, Ray goes missing. A week later Ryan finds a message from Palmer along with one of his size-changing belts, asking the youth to come find him in the Microverse because he got stuck there when exploring a change in time and space. After receiving another scolding from Dean Plumm, Ryan heads back to the lab, using the bio-belt that Ray gave him to travel there through the Wi-Fi. When he arrives, he is met by Batman and Lobo who are there to recruit Ray Palmer into the new Justice League of America. Discovering that Ray is missing, Batman decides to leave until Lobo asks Ryan if he wrote down various equations to update the bio-belt on a blackboard. Impressed, Lobo decides that Batman should recruit Ryan, despite Batman not wanting to put him in danger. Lobo says it's Ryan's choice, and Ryan joins the JLA and sometime later, heads to the City of Vanity, Oregon, to recruit The Ray into the team.The Ryan Choi version of the Atom appears in the series Batman: The Brave and the Bold voiced by James Sie. The Atom helps Batman stop evil sorcerer Felix Faust from opening Pandora's Box in "Evil Under the Sea!". He re-appeared along with Aquaman to save Batman from a virus created by Chemo in "Journey to the Center of the Bat!". A mind-controlled Ryan Choi appears in "The Siege of Starro! Part One", demonstrating the ability to grow to giant sizes, which he uses to prevent Batman from destroying a signal leading Starro to Earth. He also appears in the teaser for "The Criss Cross Conspiracy!", where he, Batman, and Aquaman battle the Bug-Eyed Bandit. In "Sword of the Atom!", a retired Ryan is coerced by Aquaman into helping Batman find Ray Palmer. At the episode's end, Ryan redons the Atom mantle.
Atom has a Crime Syndicate counterpart called Dyna-Mite (not to be confused with the Marvel comics character of the same name or Dan the Dyna-Mite), also voiced by James Sie. He appears in Deep Cover for Batman. Blue Bowman has him spy on Owlman.
An alternate universe version of Ryan Choi appears in Justice League: Gods and Monsters, voiced by Eric Bauza. He was seen encouraging Ray Palmer about his presentation on molecular miniaturization as seen when he holds one of Ray's shrunken horses.