Firestorm doesn't appear in the JLU series, but he appears in the series-based comics, being the most used comic-original character. If the mainstream comic origin is anything to go, Firestorm is the fusion of student Ronnie Raymond and professor Martin Stein, after they got caught in a nuclear accident. Technically, Ronnie is Firestorm since he keeps control of the fused body, while Stein acts as a sort of "Jiminy Cricket" giving advice to Firestorm.
