The Flash: 10 Best Characters Who Only Appeared In One Episode

10. Atom Smasher

The Flash's second season saw the introduction of the Multiverse as a concept, and this forever changed the landscape of the Arrowverse into the sprawling franchise we know it to be today.

Albert Rothstein/Atom Smasher of Earth-2 holds the distinction of being the first multiversal character introduced in the show, and while his appearance was merely the first in a long line of visitors from parallel worlds, it signaled a massive shift in the show's narrative direction and ambitions.

WWE superstar Adam "Edge" Copeland gave the role an intimidating physical presence (in addition to the CG enhancements) and sold the character's fear of Zoom fairly well.

The size-changing metahuman was forcibly recruited by the murderous Hunter Zolomon/Zoom to kill the Flash of Earth-1 in exchange for his safe return to his home dimension. Rothstein instead chose to hide on Earth-1 and murder his doppelganger so as to trick Zoom into thinking that he was dead.

Unfortunately, this did not work out as planned and Rothstein was forced to go on a rampage throughout Central City, which culminated in his death at the (indirect) hands of Barry and his team.