Ronnie Raymond was part of a group of demonstrators against a nuclear plant, but when he saw their leader tie Professor Martin Stein to leave him in an explosion, Ronnie refused to stoop to vandalism and murder, so he was also left there. The explosion didn't kill Ronnie and Stein, instead fusing them into a being named Firestorm, with the power of altering matter. Firestorm is mainly Ronnie, with Stein acting as a sort of "shoulder angel". When not in action, Firestorm can split into Ronnie and Stein, so they can continue with their normal lives until needed.
Batman: The Brave and the Bold
Jason Rusch was a 17 year old genius. On one fateful day, he and his dim-witted science teacher, Coach Ronnie Raymond were caught in a nuclear meltdown, caused by Dr. Double X, during an end of semester field trip to a nuclear test facility. Jason refused to leave Ronnie behind when his knee injury resurfaced. Both were fused together and granted superpowers. While Jason remained in control of his body, Ronnie's consciousness was transferred into Jason's mind but at times, he could also control the body. Jason struggled to cope with his new identity and powers but found comfort in the ability to alter the composition of any type of matter, the ultimate test of his will and imagination. After some training with Batman, the two decided to work together as a hero known as Firestorm.
The Flash (2014 TV Series)
Ronnie Raymond was a structural engineer for S.T.A.R Labs and the fiancee of Caitlin Snow. During the launch of the S.T.A.R. Labs particle accelerator, a malfunction occurred, and Ronnie entered the accelerator ring to manually redirect the explosion so that it would prevent serious damage to Central City, and in doing so, sacrificed his life in the process. At the same time, Dr Martin Stein, who had been working on the F.I.R.E.S.T.O.R.M. project, which focused on transmutation, was leaving S.T.A.R. Labs in order to escape from the potential explosion. When the accelerator exploded, Ronnie was carried by the dark matter wave, and fused with Stein. As a result of this, he gained abilities of fire and transmutation. However, the initial fusion between Ronnie and Stein was not stable, and they went into hiding until they were discovered by Dr Caitlin Snow. With help from The Flash, Cisco Ramon, Dr Caitlin Snow, and Harrison Wells, Ronnie and Stein learn to accept and control their abilities, and became the hero known as Firestorm. However, Ronnie was killed when he stopped a wormhole, though he was forcibly split from Stein. The Flash saved Stein, though Ronnie was killed. The Flash managed to find a replacement, Jefferson Jackson. Jefferson, or "Jax," worked as a car mechanic in Central City and was initially reluctant, though he eventually decided to become the new form of Firestorm.
Legends of Tomorrow
The new Firestorm eventually went on to join the Legends of Tomorrow alongside Captain Cold, Heat Wave, White Canary, Rip Hunter, Hawkman, Hawkgirl, and Atom. They traveled across time to defeat Vandal Savage and save Rip's family, though they were forced to let Rip's family die. Hawkman and Hawkgirl left the team to live in Central City. The Legends were scattered across time by Rip in order to prevent them from being killed by a nuclear bomb, but they were gathered by Steel and Green Arrow to prevent the Legion of Doom from stealing the Spear of Destiny. They succeeded with the help of Vixen, though they returned to a modern time riddled with time aberrations, or incorrect events in history. Rip, who had left the ship earlier, was the leader of the Time Bureau, a time organization. He was trying to hunt the Legends down for ruining the timeline, but the Legends escaped to fix time themselves. They saved the life of Zari Tomez, or Isis, and continued to fix the timeline. However, Rip decided to help them in order to find Mallus, an evil entity that had long been sealed away.