Stein is a brilliant and arrogant nuclear physicist who is described as a "brilliant but arrogant nuclear physicist who has sacrificed everything, including a marriage, for his work in transmutation." The particle accelerator fuses him with Ronnie Raymond (Robbie Amell) and he tries to find a way of separation before it's too late.
Garber is a six-time Emmy nominee known for countless roles and productions. His other current role is Papa Crane, Ichabod's father, in Sleepy Hollow.
At the Television Critics Association Summer Tour, Executive Producer Greg Berlanti said, "I think as we evolve Firestorm… you’ll meet Stein. We actually had him in a version of the pilot at one point on the train with Barry coming back from when he was coming back from Starling [City]. And so he’ll be there that day."