Alfred Pennyworth has been a part of the Batman mythos almost since the inception. To illustrate just how big of an impact he's had on the Dark Knight, Alfred is included in every single Batman adaptation to date. Even when other major characters like Commissioner Gordon or Robin are either left out or have reduced roles, Alfred still looms large as a central character. But following the The Dark Knight trilogy, Cracked noted how Alfred has an almost fanatical devotion to Bruce Wayne. A devotion that seems far too extreme for just a mere butler. So the theory goes that Alfred and Martha Wayne had an affair. And, knowing Alfred was the real father, that's why Martha leaves Bruce in Alfred's custody in the event of her death. There's no actual evidence to back up the theory, of course. Alfred has always been more than just a mere butler. In the comics and movies, he refers to Thomas and Martha Wayne more as friends or family than simple employers. And blood relative or not, he did raise Bruce, so that would explain the devotion.
Percival Constantine is the author of several novels and short stories, including the Vanguard superhero series, and regularly writes and comments on movies, comics, and other pop culture. More information can be found at his website, PercivalConstantine.com