While most "who is" stories around TV characters are pretty easy to summarize, the question "Who is Reactron?" is actually one that's potentially kind of fraught.
Prior to appearing as a Supergirl villain, the character had very little post-Crisis on Infinite Earths, pre-Flashpoint history...which wouldn't seem like a problem, right? This is, after all, a Supergirl show.
Except that they establish in-story that this is a Superman villain, and that he's making his way here from Metropolis. So...who is he? We might not have a lot of information. Even the pre-Crisis version is identified by the DC Wiki as being a threat who tangled with Supergirl and the Doom Patrol.
Benjamin Krullen is a guy with depression and/or personality issues. In the pre-Crisis era, he was a war criminal who actively targeted non-combatants and was stopped by a fellow U.S. soldier. In the post-Crisis, he was simply somebody who didn't know what to do with his life and joined the military because it offered direction. In both cases, he gained the power to harness energy but it came at a cost: his body was irradiated and he had to wear a containment suit.
In later years, Reactron was part of a group of radioactivity-powered supervillains who attacked Bludhaven. Later still, he was recruited by General Sam Lane -- along with longtime Superman villain Metallo and Lane's daughter Lucy, who plays a role in tonight's episode as well -- to a task force aimed at controlling Kryptonians on Earth. During an attack on New Krypton, it was Reactron who killed Supergirl's father, Zor-El.