As her birth mother and mentor, Mikoto is slightly frightened of her and attempted to please her as best she could as a child during her extensive training. Even in her dreams, she views her as a monstrous demon demanding her to kill. It makes it harder for her since she was trained even harder to become the Priestess's replacement.
Septuplets born to the high Priestess, Mikoto and her sisters trained together their entire life in order to become the perfect warriors in service to the Red Lotus. Despite all this however, there is no sympathy or even remorse for any sister who falls in battle.