Hatsumi Azuma is the mute adopted older sister of Hazuki Azuma, communicating through gestures and expressions. Despite being older, she is significantly shorter than Hazuki. Her origins lie in being a manifestation of Eve, the former guardian of the Great Library—a repository of infinite realities within books. As Eve, she periodically incarnates into different book worlds, assuming identities like Jill in an orphanage or Empress Sarara. Each incarnation invariably ends at age sixteen, dissolving into green light ("sōma"), regardless of the circumstances of departure.

Her relationship with Hazuki is central, marked by deep mutual affection that initially seems one-sided. While Hazuki expresses intense devotion openly, Hatsumi conveys her feelings subtly, such as through anonymous love letters later revealed as her attempts to express emotions despite muteness. A notable moment involves her preparing inedible pancakes which Hazuki consumes devotedly, viewing them as embodying Hatsumi's essence. In a critical confrontation, Hatsumi explicitly affirms her love for Hazuki.

Across incarnations, Hatsumi retains consistent traits: her unexplained muteness in book worlds, kindness, and vulnerability, evident in her role as Jill caring for orphans. Her fragmented existence stems from overexertion as a guardian, resulting in splintered forms like Kogechibi—a miniature version with limited speech and a fascination with fireworks. These fragments highlight her unstable connection to the library's power.

Her narrative concludes with the revelation that her cyclical existences served as escapes from her duties as Yami. The resolution involves her relinquishing this role permanently to remain with Hazuki, though the stability of this outcome is ambiguous. Throughout her journeys, she leaves emotional imprints on characters like Gargantua, who becomes obsessively devoted to her during her time as Jill, demonstrating her enduring impact across worlds.