Justice League: Every Character Confirmed So Far
7. The Flash
Confirmed By: It didn't need confirming, really, he's a Justice League member.
Plot Prediction:
The first positive hire for the Justice League, Barry Allen is most likely brought in fairly early and is star-struck into accepting Bruce Wayne's proposition. He's seeking a place in the world with his father incarcerated, and he will find it as a member of the team, which is probably why he says yes and the other two initially say no.
He's already a superhero of sorts, since his costume can be seen in the trailer, and it seems he arrives somewhat fully formed.
It would be rather a shame for the DCEU to change his origin story and have him powered by a Mother Box, but without that, he has no personal stake in the fight (though Batman arguably also doesn't), and his arc may well be about seeing the bigger picture of heroism.
Expect him not only to represent the humorous element of the film, but also for a father/son or big brother dynamic to develop between him and Bruce Wayne.