Confrere

Unfortunately, when it comes to the informant who turns state’s evidence against a confrere, the OED slaps on the obscure origin label.

Roger Bacon had little reverence for their Franciscan confrere, Alexander of Hales...

... but there was a limited number of magisterial chairs, and so, after a couple of years, the Dominican and Franciscan Masters had to stand aside for the next confrere in line.