noun. At best, a comrade-in-arms. At worst, someone to hold the holy-water hose.
Real citation: "At wit's end, not to mention utterly frazzled, Casey seeks help from Rabbi Sendak (Gary Oldman doing a bad imitation of a rabbi), a Kabbala guru who happens to know a thing or two about exorcism. After enlisting the co-exorcist aid of a basketball coach cleric (Idris Elba), the determined duo tie Casey to a gurney in an abandoned building, and get down to business. After much prolonged mayhem and spirit fleeing, it turns out that just a good swat with a crowbar will do the trick nicely."
(Jan. 9, 2009, Two Gays and a Movie, http://twogaysandamovie.blogspot.com/2009/01/jumby-wants-to-be-born-now.html)
Made-up citation: "If you think dating is hard, trying finding a reliable co-exorcist. Nothing strains a relationship quite like driving vicious demons out of innocent host bodies--and there's no summer vacation either."