Masumi Sera is a self-proclaimed high school detective and practitioner of the martial art Jeet Kune Do. After making a mostly correct deduction during a murder case (it is hinted that she made a mistake on purpose), she transfers to Ran's high school class. She seems to know that Conan is a detective.

Sera can fight at either the same or better level as Ran. She transfers to Ran and Sonoko's class at Teitan High. Since three years ago, she'd been living in America with her parents. She claims she'd rather live in Japan where she was born and raised, so she came to Tokyo and is currently living in a hotel.