Hazuki Azuma begins as an ordinary high school student maintaining a cold, aloof demeanor and a sharp, masculine way of speaking. Her defining obsession is an intense romantic love for her adopted older sister, Hatsumi Azuma, who communicates primarily through gestures due to muteness. This relationship drives Hazuki's entire existence when Hatsumi vanishes on her sixteenth birthday, dissolving into green light ("sōma") as Hazuki attempts to kiss her.

The disappearance thrusts Hazuki into the Great Library—a metaphysical repository where all realities exist as books. Guided by Ken, a talking bird, and joined by Lilith, the library's current guardian, Hazuki's sole motivation becomes traversing these book worlds to find Hatsumi. Hatsumi is later revealed as Eve, an immortal entity who abandoned her duties as the library's former guardian to inhabit various realities. Throughout her perilous journey, Hazuki exhibits unwavering determination, often ignoring collateral damage to worlds or inhabitants. Flashbacks reveal her emotional volatility, including jealousy-driven violence: she once cut Hatsumi's hand after discovering a love letter from another suitor.

Hazuki's psychological profile includes possessive tendencies, like collecting Hatsumi's personal items—recording videos, keeping childhood photos, masturbating while clutching Hatsumi's scented pillow. She rejects all romantic advances from others, discarding admirers' gifts and coldly rebuffing Lilith's flirtations despite relying on her for interdimensional travel. Her attachment manifests protectively; she intervenes when men approach Hatsumi and shields her from perceived threats.

During her travels within the Great Library, Hazuki gains supernatural abilities tied to its nature. She wields a katana with unexplained proficiency, defeats trained warriors, and demonstrates powers like soul manipulation, reality warping, and conceptual attacks. These abilities escalate to "hyperversal" or "metaversal" levels, granting acausality, regeneration, and nigh-omniscience within the library's abstract framework. Her encounters across worlds include facing alternate versions of Eve (Jill, Fujihime, Sarara), each triggering devastating emotional breakdowns when they inevitably vanish or reject her.

The narrative concludes paradoxically: Hazuki returns to her original world with no memory of Hatsumi, yet pregnant with a child destined to become Eve. This act erases Hatsumi's existence from history, underscoring the tragic cyclicality of her quest. Eve herself remains emotionally detached, viewing her multiversal travels as mere amusement.