Joe Johnson is the husband of Caroline Johnson.
During Castiel and Hannah's search for rogue angels, Joe runs into the pair. Joe, thinking Hannah is his wife, begs her to come home. Hannah tries to prove herself uninterested in Joe, going so far as to kiss Castiel in front of him, but Joe is not entirely convinced, saying Caroline "is not like that".
Although the pair leave him behind, the encounter affects Hannah and her vessel, Caroline, who felt Joe nearby and began yearning for him. Unable to keep the couple separated, Hannah relinquishes Caroline as her vessel, and allows Caroline to return to Joe.