Synopsis
At a school that takes in orphaned children and raises them to be magical weapons for war, one morning, a 14-year-old Sheena Totsuki suddenly loses her roommate. Sheena suppresses her grief and tries to act strong, until later that evening when a girl covered in blood approaches her. Despite her injuries, the girl remains cheerful, much to Sheena's confusion. The next day, the girl joins Sheena's class as a new student and introduces herself as Mimi Kagari, while Sheena finds out that Mimi is rumored to be the school's secret weapon as she is seemingly unable to die. The pair become roommates and grow closer despite their different outlooks on their situation within their school.

(Source:Wikipedia.com)