Fraternity of Order
The Fraternity of Order, also known as the Guvners, are a Faction in Planescape defined by their belief in the fundamental order of the multiverse. In a nutshell, they believe that everything exists and works based on underlying laws - a philosophy completely counter to that of the Xaositects - and are driven by their desire to understand these laws.
However, the Guvners aren't a Lawful Stupid Faction - they leave that niche to the Harmonium and the Mercykillers. No, the Guvners are a Faction of Rules Lawyers. If one knows the rules, then one knows the loopholes, one knows the by-laws, one knows how to exploit the rules. And if you know the rules to reality itself... then that makes you master of reality.
Within Sigil, the Fraternity of Order serves as the great, groaning bureacracy that keeps the city going; whilst one of their own Factional laws prevents them from creating laws, they're the guys and gals who keep track of all the existing laws, refine them, and keep everything running as well as it can in a city where angels and fiends are rubbing shoulders in the street despite mutul genocidal hatred for each other. In the practical sense, they are also one third of the judicial system; the Guvners are Sigil's legal advisers, advocates, attorneys and judges.