Superman

"So, figure out who we're killin' yet...? Aw, f***!"

Superman is a fictional superhero in the DC Universe. Though he never made a physical appearance in the Arkham games, he is referenced in the game Batman: Arkham Knight via commentary made by various thugs.

In the prequel novel Riddler's Gambit, set 5 months before the events of Arkham Knight, Robin makes a slight reference to the superhero via a memory that he has, while Harley Quinn compares Vicki Vale unfavorably to Lois Lane.

He is set to appear as a potential antagonist in the upcoming game Suicide Squad: Kill the Justice League, which takes place in the Arkhamverse.

Superman has numerous superhuman abilities, including super strength, super speed, near invulnerability, heat vision, and flight.

Reference to Superman in Arkham Knight
"You think the Bat is tough to kill? You don’t want to go up against the other guy in the cape." "That'll teach him (Batman). Who the hell does he think he is anyway, Superman?"

"You know the only thing that's missing is if that freak from Metropolis flew in here."

"So Bruce Wayne, huh? Never would've guessed. Next thing you know someone's gonna tell me Superman's really a janitor. Or no, a journalist."

Reference to Superman and Lois Lane in Arkham Knight: The Riddler's Gambit
They reminded him of some robots he’d seen in an old Superman cartoon.

“I can kill this Lois Lane wannabe just as dead with a stick as I can with an axe.”

Trivia

 * This incarnation of Superman is not the first to turn evil in a DC game. The NetherRealm Studios game, Injustice: Gods Among Us, largely revolved around that same idea. Ironically, in Injustice 2, Superman and other fallen heroes allied with remaining heroes to oppose the threat of Brainiac, while in Suicide Squad: Kill the Justice League, Superman is implied to be antagonistic to due being controlled by Brainiac.
 * This is also the case in a few other games, like Nightmare Superman from Infinite Crisis, Controlled Superman (due to Starro's spores) fought in DC Universe Online, and another Controlled Superman in Lego Batman 3: Beyond Gotham, acting as the final enemy of the game (which the Brainiac in that game is also responsible for).

Superman