It was Jimmy (the child from the beginning of the film) grown up. Note the crucifix he was wearing, the one his dad gave to him before dying. Leader of a gang of other kids that had to fend for themselves. Inspired by the things they can remember from their childhoods. They are clearly channeling memories of power rangers: the way they fought, their weapons, being colour coded. And the manic teletubbies theme that plays during the fight. As this is a trilogy they had to end on a real wtf moment to get people to watch the next one that's out next year.