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Kotori is Shidou's foster sister. Kotori is the head commander of Ratatosk which teaches Shidou about the spirit and spacequakes happening. She told Shidou how to save the spirits in a peaceful manner so as to prevent the spacequake from happening. Kotori has two modes which is depicted by the ribbon she wears. The white ribbon depicts the Imouto Mode which makes her show love for her brother. The black ribbon depicts the Commander Mode which shows her tsundere sadistic bossy nature.

Kotori is actually a Fire Spirit. To be exact, she became a Spirit five years ago. The reason why Shidou regenerates his wound and comes back to life is because Shidou sealed Kotori's power inside him five years ago. It was around that time that there was a third entity which was said to be the cause of Kotori becoming a Spirit and the cause of Mana