Mary currently lives in a hotel with Masumi Sera, and according to Conan she appears to be middle-school aged. Little is known about Mary's background, though she appears to be very cautious, especially with regard to meeting other people.

Her fullname is Mary Sera. Despite her appearance, she is actually a middle-aged woman; wife of Tsutomu Akai, and the mother of Shuichi Akai, Shukichi Haneda, and Masumi Sera.