The Banken (番犬, lit Watchdogs.) is a unit sent from "Tokyo" to assist in security measures regarding the Irie Institution. They consist of trained professionals in military combat from the SDF and are intended to be called only in extreme circumstances. They have access to a wide variety of military supplies and vehicles, whereas the Yamainu's supplies were very limited.
In the novel, Okonogi laments on how he would rather be in charge of the Banken instead of the Yamainu for several reasons; one being the Yamainu's incompetence, and the other being the fact that the Banken engage in much more combat situations than the Yamainu do. Okonogi's tendency to think carefully about a situation instead of jump in and be ready to engage in combat is the reason why he was assigned to the counterintelligence squad.
The Banken only appear during the ending of Matsuribayashi-hen and Miotsukushi-hen, just as the Yamainu are considering retreat and soon surrender. The combat forces rush in and overtake the Yamainu, making them willingly surrender. They were called in by Tomitake Jirō after his escape to Okinomiya. In Matsuribayashi, the Banken surround Takano in order to arrest her, but Tomitake interupts them. In Miostsukushi-hen however, they acctually try to shoot Takano, but Hanyū takes the bullet and saves Takano.