WARNING: The following contains major spoilers for Justice League: No Justice and Justice League #10-20, on sale now.
When we were first introduced to Jarro in Justice League #10, fans were wondering exactly who or what was Batman's new sidekick. It turns out that, following Starro's death at the hands of an Omega Titan in Justice League: No Justice, the Dark Knight found one of its spores and decided to let it grow in a jar, raising the offspring on pure, unadulterated justice.
But as is the case with any child, it could always lean to the tendencies of its parent. There's a reason Starro's been known as "The Conqueror" in so many villainous battles against the Justice League, and it does leave us wondering if Jarro is indeed fated to follow its footsteps, or if the baby starfish truly is a hero in the making.
THE SCIENCE OF JARRO
Starro used its spores to latch onto victims before establishing a telepathic link to control them. That's how it was able to lead its species (it's asexual, of course) on several missions to try to take over the cosmos. Basically, Starro was extending and imposing its will via this mental link, which is something we see with starfish in the ocean in their very movement.
Once one of a starfish's five arms is attracted to something, it overpowers the others thanks to its sensory and motor system. It may not have a brain per se, but this setup does act as a central nervous system, allowing the starfish to move towards whatever it likes. In Starro's case, that's ruling the galaxy. Because of its thin body wall, its mouth that loves opportunistic feeding like a vacuum cleaner and the photoreceptor cells on its body, basically it senses out opportunities to invade and conquer, and transfers its very essence to its victim -- consuming the mind of the intended target.
But if there's no target, what happens? In Jarro's case, because Starro was killed, when Bruce found the starfish, he inherited a blank slate to mold. Seeing as starfish use the aforementioned photoreceptor cells to figure out how to react to light, rather than a physical light source, Jarro's attracted to the emotional light of Bruce.