In the world of Remnant of RWBY x Justice League, the Justice League are a group of exceptionally powerful teens who were brought together by several mysterious disappearances caused by Starro. Their leaders are the Trinity, consisting of Clark Kent, Diana Prince, and Bruce Wayne.
In Justice League x RWBY: Heroes & Huntsmen, the Justice League are a team of super heroes from Earth who wound up in a simulation of Remnant. De-aged and scattered, they joined forces with huntsmen teams RWBY and JNR to escape and return to their respective worlds.
The Justice League is comprised of exceptionally powerful teens who were brought together by Team RWBY when mysterious disappearances were caused by the alien conqueror, Starro. Clark, Diana, and Bruce were the first members to unite as apart of a group, meeting at Beacon Academy.
Ruby and Bruce journey to the Faunus districts of Vale, where they then meet Barry Allen and Jesse Quick, the former of which joining on their investigation team.
Weiss and Diana make their way to the Schnee Dust Company's Vale Headquarters, learning of Victor Stone and his origins. Victor decides to aid them on their mission.
Yang, Blake, and Clark investigate Vale's harbors when they are attacked by Arthur Curry, who is similarly investigating the disappearances and assumes they are the cause. The four are attacked by mind-controlled sea creatures, and shortly after this they make their peace and ally themselves in their detective work.
The groups would reconvene with Team RWBY and the rest of the Justice League in Vale, where they were attacked by mind-controlled enemies: Team JNPR, a Tentacle Grimm, and several Faunus who all wore distinct masks. They are rescued by the Green Lantern, Jessica Cruz. After making an escape, Jessica explains her origin and reveals the existence of the alien conqueror, Starro, and his plans of world domination to the group as she shows a hologram of the universe, Remnant included. There, she enlists their help in stopping Starro, explaining Starro's intent to conquer the world through the use of powerful Semblances and the might of the Atlas Military.
Shortly after, Arthur Curry takes the group to Atlantis, a city lost to time which is his home. There, they have a moment to rest while Arthur gathers intel from spies in Atlantis, discovering that Starro has aligned with Mr. Lloyd, and the two plan on taking the Dust weaponry to a hanger on the coast of Atlas. The group spends on last night in Atlantis to prepare and enjoy themselves, and in the next morning, set off for Atlas to face Starro.
Given how none of Team RWBY or most of the Justice League can fight in deep waters, the team hatches a plan to defeat Starro. Diana, being one of the few who can survive underwater, attacks Starro without being alerted due to being an Automaton, who does not register as human. With that, Arthur and Blake Belladonna draw Starro's forces to battle, where Cyborg disables them with his technomancy Semblance. As the fight between Starro's forces and the Justice League wages, Starro calls on Team JNPR to provide himself aid.
As the Justice League’s battle against Starro continues, a mind-controlled Team JNPR joins the fray and does battle with their friends, Team RWBY. During the fight, Jessica Cruz’s mind is taken hold by Starro as she tries to help RWBY, and she begins to attack them. As the strongest member of the Justice League, Jessica uses her powers of creation to create images of Raven Branwen, Summer Rose, Adam Taurus, and Jacques Schnee to distract Team RWBY, and the various origins of the members of the Justice League. The Trinity of the Justice League escape the fight, and lure Starro into a Dust hanger off the coast of Atlas, exploding it with Starro inside, freeing the mind controlled victims.
Starro, nearly defeated by the explosion, appears on the coast. Weakened, he loses his hold over most creatures he had mind control over. Still under mind control, Team JNPR attempts to defeat Team RWBY briefly before their mind control fades. Starro attempts one final assault, facing the entirety of the Justice League, RWBY, and JNPR, only to fail. With the alien defeated, Jessica imprisons Starro and leaves to seal him away for good as the heroes aid survivors and celebrate their victory.
With Starro defeated, the heroes return to Vale, where Ozpin congratulates and thanks the Justice League. The team goes to the Allen family’s apartment, where Bruce Wayne has provided funding for the Faunus in the area. There, they decide and actualize the idea of the Watchtower, a base of operations that can be found beyond Remnant’s atmosphere through a special Dust that Jessica Cruz has created, where the League can watch for unknown dangers and protect the world.
In Atlas, Bruce, Victor, and Weiss dedicate Bruce’s old manor to being an immense orphanage, where kids won’t be exploited. Arthur offers Jessica Cruz a gift, and the League leaves Remnant to protect the world with a goodbye to Team RWBY.
The Justice League is based off the superhero team with the same name from DC Comics.
Like in the comics, they are led by Superman, Wonder Woman, and Batman, who are also referred to as the Trinity.
Starro is the first enemy of Remnant's version of the Justice League. This is likely in reference to The Brave and the Bold #28, where the Justice League of America makes its debut and is first introduced as a team when facing off against him in the Silver Age of Comic Books.
The Watchtower is a nod to the Justice League comics as well, it is a secret headquarters for the League where they operate without the public eye on them.