[Light Novel] Hagure Yuusha no Estetika Vol.8

A mysterious portal appeared to bring chaos to the world. Monsters and demons kept appearing from that portal so an organization called BABEL was formed to fight the creatures from that gateway. A brash and delinquent Akatsuki Ousawa and his (adopted?)sister Muu was sent to attend an international high school under BABEL which specializes in fighting demons and monsters from that world.