User blog comment:B214/Volume 23 discussion blog/@comment-68.133.31.5-20170325102251/@comment-10330822-20170325110255

Its like a soldier servant, or a subordinate. Its a call back to medival feudalism. You would have the "Lord", Issei, who has knights who serve him, the retainers. The lord has other servants, but the retainers are warriors who swear loyalty to the lord. A devil like Issei has a peerage, which are lower-ranked devils that serve under him, and they pretty much serve the same function as a retainer. Roygun and Bova are essentially peerage members without being in the proper peerage.