Kuraudo Oishi is a veteran police investigator at Okinomiya who has vowed to solve the mystery of the Hinamizawa murders before his retirement, due to personal reasons. Due to his uncouth tactics and the lengths that he goes to in order to solve the mystery, he is looked upon as a nuisance by the villagers.
He approaches one of the main characters to become his informant in several arcs, and is sometimes unwittingly responsible for triggering their paranoia.