Waylon Jones - better known as Killer Croc - is a mutant criminal from DC Comics. He is a regular enemy of Batman.
Waylon Jones was born with a rare genetic abnormality known as Epidermolytic hyperkeratosis, a condition that caused him to develop hard, scaly skin like a crocodile. His mother died in childbirth and his father abandoned him, leaving him to be raised by an abusive, alcoholic aunt. As he grew up, Jones' condition caused him to take on other crocodilian traits and the poor environment he lived in had a profound impact on his psychological development. Many times in his youth, he found himself in a juvenile detention centre and even killed for the first time whilst he was inside.
For a time, Jones found work in the circus, wrestling alligators and appearing on display in a freak show. He would eventually abandon this lifestyle and turn to crime as a career in order to satisfy his violent urges. He would come to Gotham City and make a name for himself, becoming known as "Killer Croc". From then on, Croc would become a regular menace to Gotham's Dark Knight defender, the Batman.
Croc is not like many of Batman's other enemies. There's no grand theme or gimmick to his crimes and he isn't very intelligent, mostly either keeping to himself and occasionally committing acts of murder or selling himself as hired muscle for Gotham's more elite criminals.
Croc's condition has led to him developing more crocodilian traits. As well as having green, scaly skin, he has developed clawed digits, slitted eyes, a mouth full of fangs and he can hold his breath underwater for long periods of time. Croc possesses superhuman strength, including the powerful jaws of a crocodile that allow him to bite through steel chains. Over time, his mutations have become more animalistic. In various comics, he has been shown to have the mouth and tail of a crocodile and has even been able to heal rapidly, including regenerating severed body parts.
Croc has developed a deep hatred for humanity as a whole and does not consider himself human any more, giving in to the predatory animal instincts of his namesake. He has committed many brutal crimes, chief among them being cannibalism. He has eaten people alive on several occasions.