Zeiss

Not only did Phil Zeiss's parents die when he was a boy, but so did any other relative that might take care of him. So a Sicilian mobster took him under his wing, and decided the boy was cut out to be a hitman for the mob. In addition to regular old training on how to kill people, Zeiss underwent surgery to have his nervous system enhanced with cybernetics. His reflexes and vision have been enhanced, and he wears special goggles wired into his brain that let him see the world as if it's in slow motion.

Zeiss's most notable personality trait is his obsession with being the best fighter in the world, which has proven problematic. On one occasion, he was hired as a bodyguard to protect an aging Gotham mobster. Zeiss spotted Batman, also there to protect the mobster, and engaged him in hand to hand combat in order to prove himself superior. With the two protectors occupied, Deadshot waltzed in and shot the guy.

Pros: With the exception of the cybernetics, his origin and his abilities fit with the realism levels. And Batman pissing off organized crime in the first two movies makes bringing in assassins very likely.

Cons: He's a lesser-known comics assassin, and much of his appeal (and design) is based around the goggles and enhanced reflexes.

How I'd do it: I guess just eliminate the cybernetics. And just to be sure he still looks like Zeiss, give him weird looking sunglasses.

Casting Guesses: Zeiss is Swiss. I don't know of any Swiss actors, but there might be some out there. By looks alone (and Zeiss has changed his look a few times, most recently with a shaved head), I might go with someone like Jason Statham, but I'm not sure he can sound like anything but a Londoner.

Verdict: It's possible, but there are lots of assassins to choose from, and Zeiss wouldn't be at the top of my list.