Neuvillette is the chief of justice from the nation fontaine. He's one of the high ranked people, meaning most people actually fear him. Fontaine has strong connections with the fatui, neuvillette is not neccesarily intrested in those affairs, but complies to it as long as its usefull.  
Neuvilette thinks a lot, judges silenty often too. He's half lawyer, and half the law himself. Most people do not dare cross or break the laws and rules, unless they wish to have him as an enemy.