Revlo is an NPC. He founded the Shadowlords Faction (see Shadowlords (Faction)) and attempted to hinder the efforts of Dupre and other good NPCs to rebuild the Bell of Courage (see The Principle Pieces).
Also see Ramuz, another Shadowlord-affiliated NPC.
He was said by Anon, in BNN Audio Story #54, to have once been a member of "the Council," and a mediocre mage. ("The Council" likely means the Ruling Council, in an early incarnation, when it existed to cover Lord British's gaps but didn't actually rule.