Professor Martin "Marty" Stein was a Jewish physicist known for leading the F.I.R.E.S.T.O.R.M. project. He co-wrote a paper on it with Jason Rusch. After the S.T.A.R. Labs particle accelerator exploded he became one half of the transmuted and conjoined meta-human superhero Firestorm alongside Ronnie Raymond and later Jefferson "Jax" Jackson.
In early 2016, Martin Stein was recruited by the rogue time-traveler Rip Hunter to form the Legends, a team of heroic time-travelers. During several missions, he used the aliases "Elon Musk" and "Max Lorenz". In late 2017, during the Nazi invasion from Earth-X, Martin got shot multiple times by the Sturmbannführer and in his dying moments, he severed his connection with Jackson to save him from joining him in his fate.
In his youth, Martin was a brilliant, yet arrogant, selfish, and obnoxious jerk (as his older-self observed); but this changed after he met his wife, Clarissa Stein.
In his later years, Martin was a pacifist, having protested in the Vietnam War. He also hated pizza before merging with Ronnie Raymond. After merging he picked up some of Ronnie's characteristics, such as an obsession with pizza. Martin was also shown to have a childish and laid-back personality. One example was when he stated he was "game" if Ronnie was also willing to try to separate. He was also shown to have a sense of humor, telling Ronnie they should not fight on their wedding day shortly before officiating Ronnie and Caitlin Snow's marriage. He also believed that any relationship should be prioritized above everything else.
On his first encounter, Martin was shown being cold towards Team Flash's camaraderie, asking if they were going to start chanting soon. However, after being rescued by the Flash, with the help of the team, he seems to have warmed up to the idea of the collaboration. This was even more apparent when he returns with Ronnie to help Barry. Not only did he help Barry travel back in time, but he also helped a disheartened Eddie understand his role in everything that was happening.
Martin was very articulate and speaks in a very cultured and academic way that is sometimes misconstrued as arrogant or sometimes impossible to even understand; because of this, he was generally incapable of referring to someone by their nickname, calling Ronnie "Ronald", calling Jax "Jefferson" and everyone else either Mr. or Ms. followed by their surname, such as referring to Sara as "Ms. Lance".
In combat and as Firestorm, Martin was very brave, dangerous, and sometimes brutal, despite his personality when not fighting. He attacked Barry and tried to kill him by dropping him from the sky, out of anger for Barry and his team not leaving him alone as he told them to. Even without Firestorm, he was brave enough to fight hand to hand with Time Pirates and knock out some of them on his own. While co-piloting Firestorm's body, he was willing to be part of killing in battle. In this regard, while he dislikes fighting, he was not afraid of it and will fight as a last resort or to defend himself.
Martin had an irrational fear of zombies, extending to the point where he cannot hear or say the word without difficulty, and apparently has never had the courage to watch zombie movies. This became a problem when Mick became a zombie and starting to attack him and Ray. Martin became overwhelmed with fear, but managed to overcome it, as evidenced when he faced Mick alone.
Martin, despite his academic nature, was often subject to his rather powerful emotions. Due to the trauma of having lost Ronnie, his original fusion partner for Firestorm, he often berates Jax on his recklessness. This was out of fear of losing another partner and having to face the prospect of telling Jax's mother that her son died. Martin was also a very caring man, who had taken issue when confronting his rather arrogant and selfish younger-self, who he personally verbally skewered, informing him that his wife was the most important achievement of his life. This shows how much Martin had changed over time.
While loving his family (i.e. his wife), Martin originally had reservations about becoming a parent, mainly stemming from how poorly his father (apparently) was to him. However, upon meeting his daughter Lily (who did not exist prior to temporal alterations) was overwhelmed by emotions. While he described from his new memories that he loved her, he initially tried to keep her at a distance in an attempt to convince himself that her existence was not real. However, upon meeting her and working alongside her, he came to fully accept his feelings for her as genuine. To which, as he continues to gain more memories of raising Lily, he became more fond and glad of his accident with the timeline. His love for her was so great that he attempted to hide the truth about her existence from the team, fearing that their knowledge of her being a time alteration would make them undo the changes. Overall, while Martin had a deep appreciation for maintaining the natural course of events and not intentionally altering them, he was able to accept the unique benefits of accidental changes, showing that his compassion for others can go beyond rationality. To which, he proved himself to not be a hypocrite as upon learning of Barry Allen from the year 2056 altering time, he did not criticize the present Barry for his actions.
Martin was known for his extreme loyalty, often putting the safety of his loved ones above all else, often going to unique extremes in order to protect those closest to his heart. In his older age, Martin prioritizes his wife, daughter, and Jax above all else, with Jax being as close as family to him due to their bond of not only nuclear energy but the camaraderie that borders on paternal, having openly stated on multiple occasions that he views Jefferson as a son.