Mary Sera is a British intelligence operative with MI6 whose physical appearance resembles a middle-school-aged girl due to the effects of the APTX 4869 drug, despite her actual age of 53. This transformation resulted from an incident in London where Vermouth, disguised as Mary's husband Tsutomu Akai, trapped her on Vauxhall Bridge and forcibly administered the substance, causing her body to regress to a childlike state. Following this, Mary returned to her daughter Masumi Sera in her diminished form. They subsequently relocated to Japan, adopting a transient lifestyle by moving between various Tokyo hotels to maintain secrecy and avoid detection. Her family connections are extensive. She is the wife of Tsutomu Akai, whom the Black Organization considers deceased, and the mother of three children: Shuichi Akai, Shukichi Haneda, and Masumi Sera. Mary is also the elder sister of Elena Miyano, making her the maternal aunt of Shiho Miyano. Ten years prior to the present, Mary and her family encountered Shinichi Kudo during a beach outing. This event later enabled Mary and Masumi to recognize Conan Edogawa as Shinichi after witnessing his involvement in a televised case involving Minerva Glass. This recognition motivated their return to Japan to seek an antidote for the APTX 4869, believing Shinichi possessed or had access to it. Her personality reflects her intelligence background and familial experiences. She exhibits caution and mistrust, particularly towards Conan Edogawa, declining to fully confide in him despite knowing his true identity. Her interactions with her children are often characterized by a stern and detached demeanor, treating them more like professional associates than family. Historical context includes instances of physical conflict with her eldest son, Shuichi, after discovering his plan to join the FBI to investigate his father's disappearance. Despite this strictness, she demonstrates underlying care, expressing pride in Shukichi's achievements as a shogi champion and showing visible distress upon learning of Tsutomu's apparent death. She holds her British heritage as a point of personal pride. Professionally, Mary retains skills honed during her MI6 career. These include expertise in surveillance technology, demonstrated when she identified and analyzed a wiretap placed by Conan in Masumi's hotel room. She possesses advanced physical combat abilities, capable of subduing adult adversaries even in her smaller body through precise strikes to vital points—a technique Conan likened to intelligence service methods. Her agility allows her to navigate challenging environments, such as scaling balconies using thin wires. Ten years earlier, her combat proficiency was sufficient to land a knife-hand strike on Shuichi, who was already trained in Jeet Kune Do at the time. Her activities in Japan extend beyond seeking an antidote. She maintains a deep interest in the unsolved double murder case involving Kohji Haneda and Amanda Hughes, events connected to Tsutomu's investigation of the Black Organization. She specifically seeks information about Asaka, Kohji Haneda's missing bodyguard. To gather intelligence while concealed, she adopts the alias "the little sister from outside the domain" to mask her true relationship with Masumi.