Creation
Jarro was created by Francis Manapul and Scott Snyder in 2018 and first appeared in Justice League #10 – Drowned Earth Prelude during the Justice League: No Justice event.
Origins
After Jarro’s Genetic Parent Starro sacrificed themselves to save the universe in Justice League: No Justice #3 Batman acquired a genetic sample of them to study it, that genetic sample grew to become Jarro.
Batman kept Jarro with him, who started to consider Batman his father.
Major Story Arcs
Justice League
In Justice League (2018) Jarro has a vision where the Justice League is defeated by The Legion of Doom and out of fear he becomes gigantic and uses his spores to put the members of the Justice League in a trance and keep them safe.
However when Batman breaks free of the trance he tries to reason with Jarro who eventually frees everyone and goes back to normal.
Dark Nights: Death Metal
In Dark Nights: Death Metal #3 Jarro is found in a prison, New Apokolips.
He is saved by the heroes who need his powers to help them not being detected by the Dark Knights and re-enter the Dark Multiverse.