Fanlu is controlled by the Brotherhood of the Southern Route, but only in a sense. In truth the Brotherhood of the Southern Route doesn’t have experience running such a large settlement, and doesn’t have the man power to properly patrol, defend, or even enforce laws in Fanlu. The Brotherhood collects taxes from those who enter the city, provides protection for merchants who stay there, fiercely defends their own buildings and areas in the town, and occasionally acts as a police force if some ruffian really starts causing trouble in Fanlu. They also preside over property disputes within town, and disagreements between merchants, acting like a sort of court. The man responsible for these limited duties, and one of the towns very founders, is Mayor Yoritomo.
The Brotherhood protects business, and their own interests in Fanlu. In addition, they sometimes have to ride out in force to deal with some exterior threat, usually some new bandit lord stirring up trouble and raiding the trade routes around town. They really don’t have the men, resources, or specialized know how handle much else.