Killer Croc

Born with a rare mutation that made his skin green and scaly, and which grew his body to grotesque proportions, Waylon Jones was raised by an alcoholic aunt and bullied relentlessly for his appearance. He briefly worked as a carnival freak under the name Killer Croc, but his misanthropy grew as did his bestial nature, pushing him to a life of crime, increasingly detached from humanity.

Batman: Arkham Asylum
Killer Croc was one of the many inmates of Arkham Asylum at the time of the Joker's takeover. His physical condition and mental state have further deteriorated since Batman arrested him on Christmas Eve. Even more deformed and reptilian in appearance, he was imprisoned in the sewer system of Arkham where the staff fed him meat and just tried to "forget about him". When Batman was escorting the Joker through the Intensive Care unit, Killer Croc was being escorted by some of the staff (Including Louie Green), with a shock collar around his neck to keep him under control.

Croc spotted Batman and claimed that he would hunt him down very soon. He was then taken back to his lair by a number of asylum security personnel when the Joker took control of the Asylum. He saw Batman passing by a locked door adjacent to the sewer. Croc punched the door's window, claiming that he would get Batman soon.

Eventually, Batman was forced to enter Croc's lair in order to retrieve the antidote to the Titan drug, which the Joker was experimenting with on some of the patients. Batman soon learned of the antidote's location after a discussion with Poison Ivy. He was then followed by another villain, the Scarecrow, down into the sewers who threatened to dump his fear gas into the water supply, which would lead straight to the Gotham River. Before he could do so, however, Croc jumped out of the water and grabbed Scarecrow, thinking that he was a lone security guard or inmate. Batman attempted to subdue Croc by activating his shock collar with a batarang.

Croc flinched for a moment before jumping back into the water, dragging Scarecrow down with him. Batman made his way to Croc's lair, where Croc attacked him several times, only to be foiled by Batman activating his shock collar with batarangs. Eventually, Croc cornered Batman at the entrance of the lair and charged at him. Unbeknownest to him, Batman had laced the floor with explosive gel and detonated it as Croc ran across it. Croc then plummeted into the water below, vowing revenge on Batman.

After Arkham
Croc was in the Asylum sewers even after the events that happened on the island. Hugo Strange sent TYGER Operatives to lure him out using human meat. They failed but still obtained Croc and sent him in the city, where he roamed the sewers/subway. A hidden room can be found, it has a dirty, broken down matress, human skulls, and his collar. Scanning this room will help to solve one of Riddler's challenges.

Batman: Arkham City
After fighting Ra's al Ghul, if Batman throws a Remote Batarang on a switch behind a gate, Croc will burst out of the wall behind it and recognize Batman. Batman then states that he doesn't want to fight him and just wants to move through. Croc says that Batman's scent has changed and senses death on him, as he goes back to into the sewer, he remarks that there is no need for a fight, as he will feed on Batman's corpse.

Catwoman mentions Croc as an option to open Hugo Strange's vault. Catwoman and several other inmates then claim that Croc ate Scarecrow during the events of Arkham Asylum (which is left in question since Scarecrow's equipment is found).