Mary Sera, also known as Mary Akai, is a British MI6 agent. She is the wife of the missing Tsutomu Akai and mother to Shuichi Akai, Shukichi Haneda, and Masumi Sera. Tsutomu vanished before Masumi's birth during his investigation into the Kohji Haneda and Amanda Hughes murders, bringing him into conflict with the Black Organization. Approximately ten years before the present, Mary traveled to London with Masumi following a sighting of Tsutomu. Leaving Masumi at their hotel, she went to meet Tsutomu at Vauxhall Bridge, only to be ambushed by Vermouth disguised as her husband. Vermouth forcibly drugged Mary with APTX-4869, causing her body to regress to the physical appearance of a middle-school child (12-15 years old). This regression occurred within the last decade and resulted in chronic health issues, including a persistent cough. After the incident, Mary and Masumi returned to Japan. Mary deduced an antidote existed after seeing Shinichi Kudo revert to his adult form during a televised case. In her regressed state, Mary operates with extreme caution and mistrust. She directs activities from hiding, constantly relocating between hotels with Masumi to evade detection. Mary conceals critical information from Masumi, such as Shukichi's shogi achievements, and remains skeptical of Conan Edogawa's trustworthiness despite Masumi's confidence. Her family dynamics are complex: she physically clashed with Shuichi years ago over his FBI ambitions, yet she demonstrates concern for her children, celebrating Shukichi's shogi championship victory. Mary actively investigates the antidote through Masumi, instructing her to use the cover identity "the little sister from outside the domain." She analyzes Conan Edogawa's surveillance gadgets, monitors his cases, and traces his connection to the Miyano family and the APTX drug. Her primary objective is obtaining a stable antidote to regain her adult form and execute a "counterattack" against Kanenori Wakita, whom she suspects poisoned her based on his recent eye injury matching one she inflicted during their encounter. Her unresolved conflicts include pursuit by Rum (Wakita's superior in the Black Organization) and the mystery surrounding Tsutomu's fate. Her knowledge of the Haneda case remains limited, and she expresses interest in identifying Asaka, the missing bodyguard from that incident.