Mr. Quinster, first name so far unknown, is an English private inquiry agent (think Sherlock Holmes but shading to the other side of the law..) He is occasionally employed by Xerxsephnia von Blitzengaard to covertly check up on things on his side of the Channel, such as the whereabouts of Agatha. Although he professes himself honored to work for the princess, that doesn't stop him biting one of the coins she gives him.
As Seffie says, he suffers the curse of competence; he is indeed able to tap his sources inside Albia's palace and track down Agatha's whereabouts (the currently locked-down undersea dome of the Queen's Society) and then, when... encouraged.. by Seffie's Smoke Knight attendant Sparafucile, he is also somehow able to get the two women and their Mysterious Cargo through a Royal Navy blockade and into the dome itself. And then hopefully get some well-earned sleep.