Kaoru Mizuno is a member of the Shimon Famiglia. He previously attended Shimon Middle School, but he and his family transferred to Namimori after an earthquake hit their area. They chose Namimori because it is relatively earthquake free and they are invited to the Vongola Decimo's Inheritance Ceremony. He is the eldest of the 7 students who transferred to Namimori Middle School. He was grouped with Yamamoto to protect Tsuna by patrolling the sports facilities in Namimori Middle School.