Jarro

Jarro the Starro in a Jar is an alien, "Star Conqueror" who was grown and adopted by Batman.

Despite having conflict over his villainous heritage, is incredibly affectionate towards Batman who he considers to be his father. Jarro also dreams of being Batman's best Robin.

Origins
Jarro's genetic parent is Starro, an alien "Star Conqueror" best known for having been the first villain of the Justice League in publication. Starro was a giant alien starfish with telepathic abilities and as-with Earth starfish, was capable of regenerative healing and asexual reproduction via bio-fission. During the events of, "No Justice", Starro tried to redeem itself by joining with the Justice League under Martian Manhunter's persuasion. Aiding in the fight against the villainous, "Omega Titans", Starro gave their life to save the universe from the villains.

Following this sacrifice, Batman came to acquire a genetic sample of Starro which he kept in a jar for study, with the cells growing into what Batman would name, "Starro in a jar" or, "Jarro" for short.

Jarro
Jarro would be kept in the Hall of Justice and would spend much time with Batman following the Dark Knight's crippling injuries. Jarro considered Batman to be his father and was incredibly affectionate towards him, all while dreaming up being his partner in crime-fighting.

Legion of Doom
When the villainous Legion of Doom grew more powerful, Jarro became consumed by fear that the Justice League would be killed by them. As such, Jarro began releasing his mind-control spores throughout the Hall of Justice in an attempt to hypnotically force the League to stay away from the Legion of Doom.

When Jarro did this, he grew to a giant and monstrous size. Batman was able to break-through to Jarro however, telling him that they are all afraid and that refusing to succumb to that fear and impose it on others is what makes them better than the villains. When Jarro realized Batman was right, he shrunk back to his tiny size and released the Justice League from his control.

By Batman's side, Jarro informed the Justice League that they would be preparing for war against the Legion of Doom.

Powers and Abilities

 * Size manipulation:
 * Telepathy:
 * Mind control Spores: Jarro can release spores which at his will can grow into starfish-like organisms that latch onto the faces of organisms to mind-control them.
 * Regeneration:
 * Bio-Fission: Jarro can asexually reproduce via effective cloning.  Severed cells of Jarro's species are capable of regrowing themselves into new individual organisms.