Mamoru is the main character in Himatsubushi-hen. He is a young police investigator at the Metropolitan Police Department in Tokyo.
He befriended Rika while investigating a case in Hinamizawa four years ago, all due to a prophecy about his wife which came true, though he just brushed it off as coincidence.
In an untold world, Mamoru regrets being unable to save his wife and Rika, and learns martial arts to clear his mind, hoping to relive the time before Himatsubushi-hen to save them. Through a miracle, Mamoru inherits his martial art skills in Matsuribayashi-hen, using them to single-handedly defeat a Yamainu squad, and decides to save Rika from her destined death as a payment for saving his wife through her prophecy.