Ronnie Raymond is a S.T.A.R. Labs maintenance worker who is also the fiancee of Caitlin Snow (Danielle Panabaker). Ronnie has an integral role in the particle accelerator accident that turned Barry Allen (Grant Gustin) into The Flash.
In the DC Comics, Ronnie Raymond was actually only one half of Firestorm, with the other half occupied by Nobel Prize winning physicist Martin Stein. Both Ronnie and Martin were caught in a nuclear firestorm that fused them into Firestorm. Martin was unconscious during most of the accident, leaving Ronnie primarily in control of Firestorm, with Martin serving as a voice of reason inside his mind.
It remains to be seen if The CW will add Martin Stein to the show, or if the producers plan on altering his origin, but the conversations between Ronnie and Martin were considered highlights of their comic book stories. Firestorm has the ability to rearrange the atomic and subatomic molecular structures of inorganic objects, but he cannot alter living things without painful, possibly deadly, consequences.
Here's what executive producer Greg Berlanti had to say about the casting in a statement.
"Geoff Johns, Andrew Kreisberg and myself designed the pilot and series to include Ronnie (aka Firestorm) -- and to actually have someone with Robbie's talent playing the role is beyond exciting. We can't wait for viewers and fans of the character to check it out."
"I'm so excited to join forces with Greg Berlanti and The CW again on The Flash. Being brought into the DC Universe is a dream, I can't wait to get started!"