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Koyori is Mana's little sister who, at the beginning of the story, is staying in the same hospital as Aono, but is soon discharged. Whenever she calls Mana by her given name instead of referring to her as her big sister, Mana strikes her head. Koyori tries to keep Aono company whenever she can and they become good friends over the time Koyori spends in the hospital. Aono also teaches her how to make origami. Koyori is very polite to others even when not required to do so. 

(Source: Wikipedia)