Manga My Wish is to Fall In Love Until You Die

There's an orphanage which trains its charges to be weapons of war, assassins specialized in magic. At this "school" Shiina has been having a difficult time coping with her roommate's death. She completely ignores rumours, including that of a supposedly impervious student by the name of "Mimi," who is thought to be even stronger than the teachers and isn't part of any group. While taking a breather, she encounters an overly cheerful young girl covered in blood, a they chat for a while before the child is hauled away by the school nurse. Shiina thought that they wouldn't meet again, but the next day, the girl transfers into her class and cheerfully introduces herself as Mimi.