Supergirl drastically altered the background of Martian Manhunter's evil brother, Ma'alefa'ak (or Malefic), from DC Comics in season 5, episode 3, "Blurred Lines". Ironically, these changes did more to change J'onn J'onzz's character on the show than his arch-enemy's.
Malefic J'onzz first appeared in Supergirl's season 4 finale, freed from the Phantom Zone by the mysterious Monitor. Malefic immediately began trying to seek revenge upon J'onn by attacking the foster family he had built on Earth. When J'onn confronted the mysterious shapeshifter targeting his friends, he was confused when Malefic revealed himself to be a Green Martian and claimed to be J'onn's brother. This prompted J'onn to seek out the help of Kelly Olsen, who had access to the high-tech machinery needed to help J'onn get inside his own head and determine that someone had altered his mind to block any memory of him having a brother, who was a traitor to his people during the Martian Civil War.
"Blurred Lines" saw J'onn turning to more natural means to explore his lost memories, asking Nia Nul if she could use her dream powers to help him work past his telepathic blocks naturally. Together, Nia and J'onn uncovered three disturbing revelations about his family's past. First, Malefic was born with a mental disability that prevented him from connecting to the Green Martians' hive mind and a strange power that caused him to inflict pain on others telepathically. Second, J'onn's father, M'yrrn, responded to this by isolating Malefic completely, eventually pushing him to join with the White Martians in the Martian Civil War. Finally, when M'yrrn realized that his actions caused the destruction of his people, J'onn sought to ease his father's tortured soul by using his own telepathy to make M'yrrn, the few surviving Green Martians and himself, forget that Malefic had ever existed.
This origin for Malefic neatly streamlines his original origin story in the comics, while also making it better fit within the altered Martian history revealed over the course of Supergirl's first four seasons. Originally Malefic was born without telepathy and grew to resent the closeness and sense of community shared by all the other Green Martians. This spurred him to create a telepathic plague he named H'ronmeer's Curse, named after the Martian god of death and fire. The plague was triggered by psychically connecting to the Green Martian hive mind and eventually caused the infected to spontaneously combust, burning those around them in their death throes. This eventually caused the extinction of the Green Martian race, apart from Malefic and J'onn.
Strangely, while Supergirl's adaptation of this background does more than the comics did to explain the circumstances that drove Malefic to turn upon his own people, it does nothing to make him more sympathetic. Indeed, the new origin largely serves to change J'onn into a more ambiguous figure willing to do what is necessary over what is morally right, much like his comic book counterpart. How the feud between the two brothers will progress as Supergirl season 5 continues is uncertain, but it seems that there will be one less Green Martian in the universe in the end.