Jax-Ur may not be as famous as General Zod, but this Kryptonian villain has been a thorn in Superman's side for almost as long. Over the years, however, the mad scientist has taken different forms. About the only thing consistent across every interpretation of Jax-Ur is that he is a scientist. In theory.
What makes Jax-Ur so fascinating from a historical perspective is how writers have found so many unique roles for the character over the years. When established villains like Lex Luthor or Brainiac are radically re-interpreted, fans enter panic mode. But Jax-Ur has never been consistent across decades of his -- or more recently her -- history!
Pre-Crisis Jax-Ur was an Unfortunate Murderer
Jax-Ur started his reign of terror during Krypton's reign. A scientist, Ur had ambitions to overthrow Krypton's government, so he started developing nuclear weapons that he would stockpile until he could use them in a coup. Only thing is, in order to see if the rockets worked, he had to test them. Depending on which Pre-Crisis story you read, Jax-Ur either tried to hit a space rock, only to accidentally hit Jor-El's space ship (yes, Superman's dad Jor-El) or he launched the nuke into one of Krypton's colonized moons. Either way, somehow the moon gets destroyed, everyone on the moon dies, and Jax-Ur is responsible for it. He is sent to the Phantom Zone for eternity.
Now, back in the Pre-Crisis Era, Phantom Zone villains would regularly escape or influence Kal-El's life, causing a mess of problems. Jax-Ur, the self-acclaimed "Worst Criminal in the Phantom Zone," would often try to cause trouble for Superboy (young Kal-El). Problem is, while Jax-Ur had some powers, he wasn't exactly physically fit, so Superman often beat him very easily. However, Jax-Ur had some muscle who helped him out: the banished Kryptonian General, Dru-Zod.
Post-Crisis Jax-Ur was More Insidious
Jax-Ur didn't appear for quite some time in the Post-Crisis continuity. While guide books and statements implied that Jax-Ur did exist, he wasn't really relevant until Geoff Johns' run on Action Comics. In it, he is a scientist who blows up a moon, like before, but here it's entirely accidental in an attempt to establish interstellar travel. The disaster also draws the attention of Brainiac; more specifically, it draws his attention to Kandor. For this, Jax-Ur is sent away to the Phantom Zone
That would have been it for Jax-Ur until the reintroduction of General Zod during the New Krypton arc. Zod unleashes several Kryptonian villains from the Phantom Zone in an attempt at a military take-over -- and among those criminals is Jax-Ur. Jax-Ur pretends to be a STAR Labs scientist, but, in actuality, is trying to uncover the secrets surrounding Flamebird and Nightwing (no relation to Dick Grayson). These two appear as guardians of the Phantom Zone, who attempt to return the criminals to it. So, essentially, Jax-Ur is tasked by Zod to keep the Kryptonians from returning to the Phantom Zone. It's almost a sympathetic portrayal, if not for some of the more insidious, underhanded tactics Jax-Ur pulls.
Jax-Ur in Superman: The Animated Series was just Zod
The DCAU's Jax-Ur is nothing like the character in the comics. But he is a lot like another Phantom Zone-trapped Kryptonian: General Zod. In Superman: The Animated Series, Jax-Ur occupies the same role Zod plays in Superman II, only here voiced by Ron Perlman. He attempts to take over the world, which forces Superman to fight and eventually beat him.
The Jax-Ur of Smallville is Dax-Ur
Jax-Ur does not appear in Smallville necessarily, but Dax-Ur does, and he is so similar to Jax-Ur, they might as well be the same character. He too is a scientist who causes horror and dread back on Krypton, only instead of blowing up a moon, he created Braniac. Dax-Ur has lived for over a hundred years thanks to Blue Kryptonite, which he believes makes him immortal. As proven after a reunion with Braniac, however, Dax-Ur might've overestimated his immortality.
The Man of Steel Jax-Ur Was an Ill-Fated Accomplice
Jax-Ur had a small role in Man of Steel. He is one of General Zod's various lackeys, serving him on his mission to create a new Krypton on Earth. Jax-Ur realizes that Jor-El grafted Krypton's Codex into Kal-El's cells, so that, in order to restate Kryptonian society, Zod would need Kal-El's body, dead or alive.
He never engages in a fight with Superman, and, like many of the escaped Kryptonians, is sucked into the Phantom Zone at the end of the film, presumably with no way out.
The Jax-Ur of Krypton is a Radical Terrorist
The most recent appearance of Jax-Ur is in the show Krypton. This might be the most radically different interpretation of Jax-Ur so far. This time, Jax-Ur is not a male scientist, but a female terrorist leader. As head of the terrorist organization Black Zero, she is responsible for many of the crimes that plague Krypton. To say any more than that would spoil the twists ahead in the series, but rest assured: there is no version of Jax-Ur remotely like this one... and that, for storytelling purposes, is a good thing!
Returning in 2019 on Syfy, Krypton stars Hannah Waddingham as Jax-Ur, Cameron Cuffe as Seg-El, Shaun Sipos as Adam Strange, Georgina Campbell as Lyta-Zod, Elliot Cowan as Daron-Vex, Ann Ogbomo as Jayna-Zod, Rasmus Hardiker as Kem, Wallis Day as Nyssa-Vex, Aaron Pierre as Dev-Em, Ian McElhinney as Val-El and Blake Ritson as Brainiac.