Dresden Files - It's a Small World
It's Joe, duh.
Once a member of a occult group learning their magic from a vile demon. Joe was hooked on black magic, using the magic to forward nefarious means. Luckily Dean tracked down the cult and sent the Demon back to the depths of the Nevernever. And Dean seeing salvation in Joe helped him to cleaning up and saving himself.
Joe now lives with Dean and STAYS AWAY FROM THE GARAGE.
Dean can always rely on Joe to either fix up his Bike or come pick him up when it breaks down, or do a doughnut run for summoning Rombo.
(MORE TO BE ADDED LATER)