Harold Allnut is a mute hunchback with a gift for building and repairing electronics, machines, and computers. He was never an incredibly important part of Batman's life or anything, but he had some nice moments along the way.
In his introduction to Batman's world, the Penguin discovered Harold's talent for technology, and forced him to build devices to aid in the Penguin's schemes. After taking care of the Penguin, Batman recognized Harold's genuis, and recruited him to work on Bat-gadgets. Harold took up residence in the Bat-Cave for many years, repairing and building all kinds of things. Though he basically never left the cave, he seemed happy to have a purpose in life.
In No Man's Land, a terrible earthquake destroys most of Gotham, and since Batman would not be using the cave for some time, Harold went out into Gotham for the first time in years. He was shocked to find the city falling apart, and went to work on anything in sight that needed fixing.
Later, in the Hush storyline, Hush repairs Harold's spine and his voice and Hush forces him to betray Batman. Harold confesses to Batman, but before he can reveal Hush's identity, Hush kills him. Until that point, no one knew Harold's last name, so Batman had to research his life to give him a proper headstone at his grave.
Pros: Although Wayne Enterprises supplies all of Batman's equipment in the movies, someone would have to do maintenance on the Batmobile, and I can't imagine Bruce just bringing it back for repairs. Having Harold around would at least explain how the repairs happen.
Cons: Harold's unexplained ability to work with technology doesn't make a whole lot of sense, and only works in comic books because those kinds of things always happen in comics. Also, I'm not sure if it's politically correct or whatever to have a mute hunchback servant guy.
How I'd do it: Have a guy named Harold who's an excellent mechanic blackmailed into working for a villain, Batman rescues him, and sets him up with a secret garage where he'll bring the Batmobile (or pod or whatever else) for repairs. I don't think he needs to be in the cave, know Batman's identity, be a mute with a spinal deformity, or any of that.
Casting Guesses: It could literally be anyone, so I'll say Luis Guzman, since he's great at everything.
Verdict: The Batmobile didn't make it out of the Dark Knight in one piece, so it's possible there could be a Harold-type character.
After The Scarecrow, Ra's Al Ghul, The Joker, and Two-Face, who should be Batman's next enemy in the Batman Begins/The Dark Knight series? Well, as of now we know that Tom Hardy will play Bane and Anne Hathaway will play Catwoman.
Check out the master list for all the villains covered. I've pretty much run out by now. Obviously, there are more Batman villains, but not any of significance that I know of. As of February 2009, no one is confirmed as a villain. Not the Johnny Depp Riddler, not Phillip Seymour Hoffman Penguin, nobody.