Mamoru Miyanomori, writing under the pen name Mimori Mamoru, is a 25-year-old novelist often frequenting Ginza. Born June 11, he has authored romance novels specializing in tragic love stories for five years. Though he claims financial hardship, his situation is less severe than portrayed. His long black hair is usually tied in a ponytail, complementing his black eyes. He dons distinct attire for different occasions: a military uniform during the prime minister's address, a black suit at parties, and a hakama outdoors.

Secretly the illegitimate son of the Miyanomori family, Mamoru harbors a deep grudge against patriarch Genichirou Miyanomori and his legitimate sons, stemming from a troubled childhood. This forged him into a suspicious, cautious, and vengeful individual who shows no mercy to his targets. Proficient in marksmanship, disguise, and stealth infiltration, he gathers intelligence on the family, memorizes their mansion blueprints, and plots to assassinate Prime Minister Saeki—Genichirou's ally—during Saeki's public address on September 25.

Mamoru's interactions with Haru Asagi, a Miyanomori family maid, become pivotal. Initially manipulating her during chance Ginza encounters to extract family information, he unexpectedly develops genuine affection. He demonstrates uncharacteristic kindness towards Haru, aiding her in canceling an arranged marriage through an elaborate ruse and expressing concern for her well-being. This relationship catalyzes his abandonment of the revenge plot. He confides in Haru, revealing his mother's early death, fatherless upbringing, and the resulting resolve for vengeance.

His personality blends sternness with underlying kindness, typically reserved for Haru. He appears clumsy in daily life yet remains calculating in schemes. Writing serves as a creative outlet, and he enjoys dogs while disliking cats. Following the failed assassination attempt, influenced by his bond with Haru and its emotional complexity, he reconciles with his identity and halts his vengeful pursuits.