Aya Hiraga

She is a genius of Amdo division, and as such she is enlisted by other students to upgrade or modify their weapons. Kinji's Beretta pistol is one such example. Her ancestor is Hiraga Gennai. According to Kinji, her labor fees are considerably expensive. Despite being a genius in customizing weapons, sometimes due to sloppy work, the guns she works at has a probability to malfunction. Kinji states that considering her skills, she should be ranked S. However, due to the massive fees she naively charges for her services, coupled with the large number of illegal modifications she performs, she is ranked A. (Source: Hidan no Aria Wiki)