Jake Donaldson was one of the best Detectives on the force; sharp, alert, and with great powers of deduction. It was really only a matter of time before he became exposed to the Supernatural.
Jake's great strength and flaw as a cop was that he couldn't let a case go. He had an almost perfect record of closed cases, thanks entirely to his dogged refusal to stop working the clues on even the most apparantly unsolvable of mysteries. Yet when he encountered his first Supernatural case, a curious locked door murder scenario, he was stymied. The facts of the case required the existence of things that clearly could not actually exist...and yet there it was. He didn't solve that one, but a few months later when a similar case occured involving a man killed inside of a secured safe room with no way in or out and surveillance video that strangely "fuzzed out" the critical few minutes in which the deed was done, Jake refused to just let the case file slide onto the "unsolveable" pile.
Turning all his abilities and experience to the task, Jake eventually made his case...but there was no way he could turn it in or move forward with it officially. Now, Jake had heard other older cops talking from time to time about "weird stuff" cases, but he'd always blown it off as nonsense. However he now had evidence of it for himself, for the culprit he had successfully hounded down appeared to be some kind of evil apparition...i.e., a ghost!
Furthermore, it seemed to have been 'summoned' and controlled by some kind of weird cult who had what appeared to be genuine supernatural abilities! Jake closed the case, but the entire thing got handwaved away in a storm of jurisdictional bullshit by some odd FBI Agents and a civilian consultant going by the name of Dr. Allosius Jones.
Jake's eyes were now opened...perhaps a little too far. Faced with the new knowledge of the esoteric things he had previously not believed in, Jake started seeing Supernatural doings behind even the most mundane of crimes. His ability to solve crimes was impaired, and eventually he was put on a suspension without leave or pay which he's just never returned from.
Instead Jake opened a private detective agency, got licensed as a Hunter with Section M, and he now specializes in the sorts of "weird cases" other PI's don't want to deal with. They don't all have a supernatural angle, but some do, and those are the ones Jake looks foward to.