Clayface

Initially an actor in horror films, Basil Karlo went mad when he learned a classic film of his was to be remade with a different actor in the lead role. He took on the mask of the film's villain, "Clayface", and killed several of the remake's cast and crew before being stopped by Batman and Robin.

Batman: Arkham Asylum
Batman, while searching for Arkham Warden Quincy Sharp, came across a cell behind a clear wall with no bars. A sign next to it read "Danger. Do not approach. Inmate may be impersonating other personel." The cell contained a female mannequin, white hand-prints, and excess Clayface goo. What appeared to be security guard Aaron Cash was pacing behind the wall, urging Batman to let him out of the cell.

However, after rescuing the warden in the room above, another man looking like the warden appeared in the cell, claiming that the one rescued was an imposter and begged to be let out. When Batman turned around, a morphing noise sounded and in the cell appeared what seemed to be Commissioner Gordon. He simply laughed at Batman. Detective Mode revealed that the shapeshifter had no skeleton, and scanning him revealed him as Karlo/Clayface. His profile could then be read and the Riddler's riddle, "A case of mistaken identities?" was solved. Clayface commended the detective for discovering his identity, and asked him to keep it a secret, as he had a reputation to keep.

After all but one of the hidden chronicles are found around Arkham, Quincy Sharp has disappeared from his santuary. Clayface will mention how the warden left in a hurry despite Batman's insistence that he stay hidden, all while impersonating Commisioner Gordon.

After Arkham
When the rescue teams of the GCPD swept through the ruined Asylum after Joker's plans, one uninformed guard believed that Sharp was trapped within the same cell Batman saw and let him out, while in reality it was simply Clayface taking advantage of the situation to finally escape. It was only after the real Warden Sharp was found did the guard realize his lethal mistake, but Clayface was long gone by that point and at large.

Basil Karlo enters Arkham City completely undetected. Assuming the guise of numerous inmates and security personnel alike, the monstrous mudpack eventually makes contact with the Joker, who is revealed to be his cryptic employer. At first, Karlo is reluctant to form an alliance with the dying clown, but is quickly convinced to aid the Joker and embroils himself in the Clown Prince of Crime’s last megalomaniac master plan.

Batman: Arkham City
Despite not actually being imprisoned in Arkham City, Clayface has sided with the Joker for one key reason; to play “the role of a lifetime”. At the request of the Joker, Karlo poses as the healthy crazed Clown Prince of Crime and willingly carries out the vast amounts of chaos the actual Joker would if he was capable. Clayface interacts with Batman on numerous occasions, and countless others including Talia al Ghul, managing to fool and suede all within the city into believing that the Joker had made a full recovery. Until it was far too late, Joker and his ‘twin’ even managed to hide the deadly yet truthful nature of the clowning duo from Hugo Strange, Batman, and for a brief period of time, Harley Quinn.

With Clayface’s vast and nearly unstoppable abilities, the Joker knew he had all but conquered Arkham City. Next to doubling as him, the Joker used the feat of clay as a puppet of power to carry out his most extreme tasks; having the shape-shifter shut down the Black Mask’s base of operations (opening up the Sionis Steel Mill to Joker and his gang) and ultimately convince a cold tempered Mr. Freeze to develop a cure for the clown’s growing illness (although Hugo Strange had already taken Fries’s wife, Nora, hostage by that point). The Joker was almost sure he was clearing a pathway straight to his salvation and with the spotlight focused on his doppelganger he knew that he could easily stay hidden in the shadows long enough to see through his final act; which coincidentally would also be Clayface’s final “performance” as well.

The plan was not realized by Batman until Talia was taken prisoner by the Joker at the Monarch Theatre. However, Talia managed to free herself and disarm the Joker, proceeding to stab him. Not long afterwards did Batman remember something that had been said to him previously and pieced together that Joker had been impersonated the entire time.

The actual, Joker, his illness continuing to eat away his flesh, revealed himself and shot Talia dead on the spot after the ruse had been seen through. Clayface arose and took on his natural form, a hideous, monstrous mass of mud that Batman was then forced to fight if he was to retrieve the only remaining vial of the cure. He primarily used Mr. Freeze's cryogenic explosives to slow Clayface down, although he thawed when he tried to get the vial out of the hole. Clayface eventually was frozen solid by the explosives, causing Batman to use Talia's sword (which was still impaled on him) to cut his frozen body up.

He was eventually thawed after the Joker blew up the floor of the theater to give Batman a "change of scenery" to "help ease the pain" (referring to Talia's demise), although the process also caused him to be unable to retain his natural form outside of a half-melted appearance, and resorted to utilizing miniature Clayfaces to attack Batman. After a long and exhausting struggle, Batman was finally able to disable his enemy into the Lazarus Pit Chamber beneath the Theatre. The aftermath of the ensuing battle left the Monarch in complete ruins.