I'm thinking my guy would be a heavily prepared young guy, a huge nerd with experience of the supernatural from a childhood incident. He's seen as someone who could have been successful or married by now, but he spends all of his time on his research and on training for his hunts and investigations. He decides to join the Sunday Group to give his expertise. He'd serve as an 'expert' first, and 'officer in the field' role, though badass lady cops are first when it comes to that ;)