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Hatsumi is Hazuki's adopted older sister and love interest, albeit Hazuki's love is one-sided. Hatsumi appears to be mute (though it is proven otherwise on several occasions) and communicates mostly through gestures and facial expressions. Despite being older, Hatsumi is much shorter and thinner than Hazuki. Childhood memory flash-backs show Hazuki as a small, sickly child who might not have lived if not for Hatsumi's care.

Although it seems that Hazuki's love is one-sided, episode seven shows that Hatsumi does have feelings for Hazuki and was trying to express herself as best as she could with the knowledge that she cannot stay beyond her sixteenth birthday (suggestions of this were shown along the way by her sending Hazuki anonymous love letters — later expressed in full and in words toward the end of the anime). In the first episode, Hatsumi disappears in a burst of green light ("sōma") at midnight on her sixteenth birthday, as Hazuki tries to steal a kiss in her sleep. The events that follow lead Hazuki to a mysterious library in another dimension, in which all the worlds are kept in books.

Hazuki learns from the Library guardian, Lilith, that Hatsumi is only one of the forms of the previous guardian (Eve), and that Eve stays in different books for fun until she turns sixteen.