Waylon Jones was born with an extreme form of the medical condition "Epidermolytic Hyperkeratosis", which caused his appearance to develop progressively into that of a crocodile, hence his name. As a teenager, Waylon had no friends, and was the object of ridicule of those who knew him. He found a job in a carnival sideshow ... wrestling alligators. It was at this point that Waylon began referring to himself as Killer Croc. Even then, his strength level was greatly beyond that of a normal human. Croc realized that there was more money to be made in crime, so he set out to become Gotham's most powerful underground figure.
When the Justice League arrive in Gotham to remove the patients at Arkham Asylum, only to meet resistance from Batman, Killer Croc observes the argument between the heroes in silence until Cyborg angrily attacks Batman after discovering the virus he had uploaded into him during their first meeting, with Croc commenting that as "Pretty evil." When Harley Quinn releases all the inmates to attack the heroes, Croc goes straight for Batman, preventing the Dark Knight from coming to his son Robin's aid as he's dragged underground by Solomon Grundy.
Injustice: Gods Among Us
Killer Croc appears in the Arkham Asylum stage. He is in the recreation area with three other Gotham villains, and beats the player/opponent up if they're thrown into the room.
He also appears in one of Batman's S.T.A.R. Labs Missions where he frees inmates at Stryker's Island and Batman must defeat both the inmates and Killer Croc, while dodging Croc's Shoulder Charge attacks.
Injustice 2 Comic
Killer Croc was seen as one of the members of the Suicide Squad. He and Orca kill Ra's hostages, including Ted Kord.
Killer Croc now lives in the sewers running through the Batcave in Injustice 2, for some reason. He assists the player character during the stage transition by smashing the opponent numerous times into a wall before the wall breaks.
Powers and Abilities
Croc is about as strong as his physique would suggest, even if his strength is nowhere near the levels of other metahumans like Superman. He also has teeth, strong enough to bite through steel and human flesh. Croc also possesses an immense healing factor, able to heal himself faster than a normal human's healing. His skin has also hardened to the point where he is practically immune to abrasion or even bullets. Though not as proficient as Batman, he is a capable fighter, skilled in wrestling and street fighting. His speed and agility is also impressive. True to his name, he is also an adept swimmer, able to hold his breath for some period of time.
Killer Croc is a massive human mutated into a reptile like appearance, complete with dark green scales all over his body with a light green color on his stomach and long claws jutting from his fingers, with only a pair of orange prison pants and chains attached to his wrists serving as a form of clothing.