Sheena Totsuki is a 14-year-old orphanage student training to become a magical weapon for war. Her magical abilities and academic performance are consistently average. She rejects warfare and harming others, harboring deep doubts about the orphanage's mission and her place in it. The death of her first roommate profoundly affects her. Soon after, she encounters Mimi Kagari, a girl covered in blood but unnervingly cheerful, shortly before Mimi becomes her new roommate. Initially disturbed by Mimi's carefree attitude towards violence and rumors labeling her the school's immortal secret weapon, Sheena keeps her distance. Despite reluctance, she assists Mimi with daily tasks like brushing her hair and finding clothes, recognizing Mimi's lack of socialization and childlike dependence. Sheena actively works to instill in Mimi the value of all life, including enemies, gradually humanizing her. This forges an inseparable bond, transforming their relationship from caretaker-dependent to mutual romantic partners. Sheena shares her first kiss with Mimi, who uses it to transfer healing magic. Sheena possesses rare recovery magic, which she uses to ease Mimi's pain during post-battle reconstruction. After discovering Mimi hides injuries to avoid upsetting her, Sheena confronts her, causing a temporary rift. They reconcile when Sheena commits to unconditionally supporting Mimi, even accepting her bloodied state after battles. Their relationship deepens through gestures like wearing matching collars, head caresses, embraces, crafting daisy crowns, and going on dates. Sheena's defining wish is to love Mimi until her own death. She aspires to become a traveling doctor to follow Mimi to battlefronts. She is skilled at making onigiri, which first attracts Mimi.