SPOILER WARNING: This article contains major spoilers for "Event Horizon," the Supergirl Season 5 premiere.
Martian Manhunter has a brother that even he didn't know about. In "Event Horizon," the Supergirl Season 5 premiere, Ma'alefa'ak revealed himself to J'onn after unleashing the villainous Midnight on National City.
When J'onn returned home after Kara's Pulitzer Prize reception, he found a little girl in his office -- the same little girl, in fact, who stole Superman's Kryptonian escape pod. He asked her if she needed help, only for her to transform into a Martian before his very eyes.
"You're the shapeshifter who released Midnight," J'onn realized. "What do you want from me?"
"To be punished for your crimes," Ma'alefa'ak spat back. "You may have defeated Midnight, but I'll make sure you get what you deserve."
"Why? Who are you?" J'onn demanded.
"Don't pretend you don't recognize me, brother," he replied, removing his hood.
J'onn was extremely taken aback. "I have no brother," he insisted.
"Lies!" the Martian cried. "You and Father are more evil than I knew, but justice will prevail! I will do to you as you did to me."
"You say you're my brother? Prove it!" J'onn yelled, launching himself at Ma'alefa'ak. They tussled for a moment, only for a blast of rainbow-colored light to push them apart. Overwhelmed, J'onn was knocked unconscious on the floor of his office.
Ma'alefa'ak made his Supergirl debut in the Season 4 finale. In the episode's stinger, the Monitor ushered him into National City. He told Ma'alefa'ak he could exact revenge on his brother J'onn at last. At the time of writing, it is unclear why Ma'alefa'ak wants revenge.
Given the fact J'onn doesn't recognize him, Ma'alefa'ak may even be from a different world. After all, the Monitor has the ability to pass between alternate universes. Indeed, Ma'alefa'ak may even be part of the Monitor's plan to test Supergirl's Earth. As it stands, the Monitor only tested Earth-1, Barry Allen and Oliver Queen's world, in the "Elseworlds" crossover.
Airing Sundays at 9 p.m. ET/PT on The CW, Supergirl stars Melissa Benoist as Kara Danvers, David Harewood as Martian Manhunter, Mehcad Brooks as Jimmy Olsen and Chyler Leigh as Alex Danvers.