The director's cut of Justice League features a lot more footage filmed at STAR Labs, fleshing out stories not only for Silas and Victor Stone, but also introducing fans to Ryan Choi, Silas's right hand and the man who has to run STAR when Silas is off the board. The character, played by Zheng Kai, is actually a costumed superhero in the comics. As one of a number of heroes who went by the name of The Atom over the years, Ryan could be seen as DC's version of Ant-Man (although The Atom came first -- but if you know that, then you probably don't need us to explain who Ryan Choi is, whereas casual fans are more likely to have seen the Avengers movies).
In Zack Snyder's Justice League, Choi doesn't really give any inkling that could be in his future -- although he is an expert in nanotechnology, which makes sense for a shrinking hero. This marks the character's feature film debut -- and would have been his live-action debut if he had appeared in the theatrical cut of Justice League. Instead, he's now the second live-action version, behind Osric Chau's Choi, who appeared in the 2019-2020 CW event "Crisis on Infinite Earths."
Prior to his becoming one of the Paragons that would help keep the multiverse safe during the Crisis, Choi had been mentioned on an episode of The Flash, referenced as a scientist in the future who engineered Barry Allen's Flash ring and costume. In comics, Ryan was created by Gail Simone and Grant Morrison, and first appeared in DCU: Brave New World #1. He's the longtime protege of Ray Palmer (Brandon Routh's character on DC's Legends of Tomorrow) and, after Palmer disappears, Ryan moves from Hong Kong to Ivy Town and takes over Ray's place teaching at Ivy University. Once there, Ryan follows Ray's clues and discovers a "bio-belt" and becomes the new Atom.
"I don't usually read comics. The Ryan Choi comics were actually the first I read as a kid and I only read it because my roommate - this was like seven, eight years ago - my roommate, we went through a list of all these superheroes I could play," Chau said in an interview ahead of the "Crisis" event. "If you could play a superhero it would probably be Ryan Choi. Then I got all of Ryan Choi comics and I was like, 'Yeah, you're right.'"
"And this was like way back when superheroes were just starting to become prominent, and as this Asian actor I was like I got to do something towards that," he continued. "We actually put a pitch packet together for Ryan Choi because I did a lot of research and I love the story. I even connected with Gail Simone who is the writer of it. So, when I came to the audition - they're were fake sides, but I knew right away I'm like, 'This is Ryan.' Like I told the writers in the room like 'This is Ryan, right?' I think I definitely caught them off guard."