Masumi Sera, a sharp-witted high school detective, enrolls at Teitan High as Ran Mouri and Sonoko Suzuki’s classmate, driven by suspicions surrounding her FBI agent brother Shuichi Akai’s alleged death. Born into the covert Akai-Sera lineage—her father Tsutomu vanished mysteriously, her mother Mary hides in hotels alongside her, and her siblings include shōgi champion Shukichi Haneda—she navigates a fractured family legacy steeped in espionage. Masumi’s boyish charm and sociable nature shine through her casual use of the masculine pronoun "boku" and a laid-back wardrobe of hoodies and jeans. Her short dark hair, tomboyish fashion, and understated physique frequently spark gender confusion, which she deflects with humor—playfully contrasting her own frame with her mother’s curves while privately yearning for physical maturation. Though quick to laugh, she bristles at remarks about her flat chest, revealing vulnerability beneath her confident exterior. As a sleuth, she combines sharp deductive instincts with adaptable strategies, dismantling culprits’ schemes through direct confrontation rather than theatrics. Her fearless pragmatism extends to calculated risks, even deliberately positioning herself in a sniper’s crosshails to resolve a hostage crisis. Trained in Jeet Kune Do, she rivals Ran Mouri’s karate prowess, a rivalry born during a bus brawl sparked by mistaken identity. Her transcontinental upbringing across England and America anchors her global perspective, while her fandom for Yusaku Kudo’s "Night Baron" novels and Shuichi’s signature "Case Closed" catchphrase reflect her detective ethos. The Akai family’s Gundam-inspired names carry echoes of Mobile Suit Gundam lore, nodding to her creator’s inspirations. In *The Scarlet Alibi*, her dynamic with siblings Shukichi and Shuichi and mother Mary unfolds through interwoven film clips and fresh narration, spotlighting their collective covert missions. A post-credits scene hints at impending developments tied to *The Scarlet Bullet’s* delayed release. Across episodes and films, her arc chronicles her relentless pursuit of familial truths, tactical brilliance in navigating espionage webs, and gradual integration into the core protagonist circle, cementing her role in the Akai family’s shadowy operatives.