Killer Croc's origin has undergone numerous versions. His most common origin story is that he was once a drug dealer who contracted a disease that caused him to grow green scales all over his body. Over the years, his portrayal by comic book artists has grown increasingly reptilian. His personality has also varied, ranging from a raging animal to a clever villain. Regardless of his current portrayal, he usually dwells in sewers and subways.
Batman: Arkham Asylum
Batman first sees Killer Croc while escorting the captured Joker through the asylum. At this point the Arkham staff are relocating Croc from his old cell to a new lair in the sewers below the asylum, and Croc threatens to attack Batman. Some time after The Joker develops the dangerous drug Titan, which he plans to use to create his own mutant army. Batman talks to Poison Ivy, who tells him the Titan antidote is located in Croc's lair in the sewers. Around the same time Scarecrow tries to head into the sewers to dump his "fear gas" and poison the Gotham City water supply, not knowing that Croc is now dwelling in the sewers. Batman follows Scarecrow into the sewers, where Croc attacks Scarecrow, however Batman manages to fend him off by using his Batarangs to activate his shock collar. Batman makes his way through the sewers, collecting the Titan antidote, and holding back Croc by using his Batarangs. When Croc finally confronts Batman at the entrance to the sewers Batman uses his explosive gel to destroy the floor and send Croc plummeting into the water below.
Batman: Arkham City
Croc appears in an easter egg if a Remote Batarang lands in the sewer.
Batman: Arkham Knight
Killer Croc appears as the secondary antagonist of the "Beneath the Surface" side mission of the Season of Infamy DLC pack. He was sent to Iron Heights Penitentiary and got experimented by Warden Ranken, making his appearance more ferocious. Batman and Nightwing caught up to him and arrested Croc and Ranken. They got sent to GCPD in separate jail cells. Croc also appears as a hallucination at the end of the main story. After Batman got unmasked, Croc would remark to Batman that he sure would taste rich now knowing he is Bruce Wayne.