Waylon Jones is born with the medical condition Epidermolytic hyperkeratosis that caused him to grow progressively into the likeness of crocodile. In the comics his intelligence level has varied from competent schemer to berserk monster.

In his original appearances, he resembled a powerfully-built man covered entirely in green scales. However, his disease has slowly robbed him of all identifiable human traits. In his most recent appearances, he has an enlongated snout and tail.

Once he was a cocaine/arms dealer until foiled by Batman and Robin.

In Batman issue 489, Killer Croc attacked a shopping mall where Batman and Robin arrived later. After Killer Croc received several punches, Batman got distracted by an image of Bane, Killer grabbed Batman and tried to break his back. Robin kicked Killer Croc right near Bane and Bane broke Killer Croc's arms. He was then put back into Arkham. However, it should be noted that Jean-Paul Valley was standing in for Bruce Wayne as Batman in this issue.

When Bane helped get everyone out of Arkham in the Knightfall saga, Killer Croc went to get revenge on Bane. While in the sewers, he smelled Bane and went after him but only found Robin. Bane then attacked him but the ledge they were standing on broke and the three fell into the sewers. The fight ended up as a draw.