Dudley starts showing signs of accidental magic about a year after Harry and, knowing that his parents loath magic, hides it. He would have become an Obscurial. But instead, Harry and Dudley make a pact, to always support and accept each other. "Brothers, always."
Harry stared down at his bleeding hand, the words "I must not tell lies" dripping down his skin in blood-red liquid. And snapped. Harry Potter decides he's had enough of the wizarding world throwing him crap. he's going to throw it right back.