Kotori Itsuka is one of the main heroines in Date A Live series. She is Shidou's foster sister and a middle school student. Kotori is the head commander of Ratatoskyr 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 ribbons depicts the Imouto Mode which makes her show love for her brother, which according to her, is that of her "weak self," a delicate little sister who is very dependent upon her big brother. The black ribbons depicts the Commander Mode which shows her tsundere sadistic bossy nature. While wearing them, she becomes her "strong self," a charismatic and sometimes cynical leader able to successfully command the airship, Fraxinus. According to Reine, the Kotori with white ribbons is the one that can honestly say how she feels, which would mean that the Kotori with black ribbons is simply putting a strong front because the situation requires so.
(Source: Date A Live Wikia)
