Mike Guenther half owned a garage with John Winchester before the fire killed Mary. After the fire, John sold his half of the garage to Mike, before disappearing.
There is a snippet more of the interview with Mike from Dean and Sam, under the "deleted scenes".
Listed in John's Journal is a wife for Mike, named Kate Guenther. Also listed was that the Winchesters stayed with Mike and his wife right after the fire, until John was able to get back on his feet.
Also in John's Journal, Mike was trying to get John to get help, that Mike thought John was nuts. Mike also said something to Kate, because Kate was also trying to get John some help.
It is questionable if John sold his half of the garage to Mike, or gave it to him. In the deleted scene Mike tells Dean and Sam that John sold Mike his half of the garage, and he started building his arsonal. But the entry in John's Journal mentions Mike ragging on John to come to work, and John not caring. John then explodes and writes if Mike cares about the garage so much, then John will just give it to him.