The Atom is unique in the sense that he can use his ability to shrink as a way of dodging attacks, provided the player times it up right. Armed with his quantum bio-belt and a "prodigal" knowledge of physics, Atom is able to shrink to the size of an atom (funny, that) and grow to big proportions.
"Physics prodigy Ryan Choi took on the mantle of the Atom when his mentor-and original Atom-Ray Palmer mysteriously disappeared".
You can check out Atom in action, below. The Atom will use his mastery of science and technology to assault his foes with stuns, snares and frequent size changes that will likely make him extremely hard to hit.
Look out Deadshot, your title of "most annoying character in the game" might be up for contention. Fighter Pack 3 will also be adding The Enchantress and all four Teenage Mutant Ninja Turtles. Watch the trailer for Atom below. He's a part of the Fighter Pack 3, or you'll get him if you bought the Injustice 2 Ultimate Edition. Fighter Pack 1 includes Red Hood, Starfire, and Sub-Zero, and Fighter Pack 2 features Black Manta, Raiden, and Hellboy.
The game is available for the PC, PlayStation 4, Xbox One and mobile.