Kuu is the youngest of the older Haibane residing at Old Home, having emerged from her cocoon roughly two years before Rakka arrives. Her name, meaning "air" or "sky," comes from her cocoon dream of floating weightlessly among the clouds. Despite being classified among the older group, she is physically the smallest and harbors a childlike heart, which often makes her resent being treated as a kid.

In personality, Kuu is defined by an buoyant, almost inexhaustible cheerfulness and curiosity. She displays a tomboyish energy, always eager to explore and to try new things, though her enthusiasm sometimes leads to misadventures. For instance, she once attempted to ride Reki's scooter and crashed into a telephone pole, and wearing Hikari's glasses made her so dizzy that she tumbled down a staircase. These mishaps never dampen her spirited nature. She works part-time at a café in town, where, humorously, her payment often comes in the form of sugar cubes. Underneath her sunny exterior runs a desire to be seen as mature and capable, and she makes conscious efforts to emulate the older Haibane, especially Reki.

Her primary motivation during her time in Glie centers on finding her place within the family-like community at Old Home. She yearns to be included as an equal and not simply the youngest. This longing is tied to a subtle insecurity about her size and age, which she gradually overcomes. By the time of her departure, she has achieved a quiet inner peace and a sense of fulfillment that allows her to move on.

Kuu's role in the story is brief but pivotal. She is the first among the main group of Haibane to experience the Day of Flight, the mysterious event that marks a Haibane's final departure from the walled town. Her disappearance serves as the narrative turning point that introduces the reality of the Day of Flight to both Rakka and the audience, and the grief it leaves in its wake becomes a catalyst for Rakka’s own emotional unraveling and later growth.

Her most significant relationship is with Rakka. The two become close friends almost immediately after Rakka's arrival. Kuu confides in Rakka that she had secretly wished for a younger sister but was ultimately happy that Rakka turned out to be older, because a younger sibling would have been assigned to the little ones’ dormitory, separating them. Kuu acts as an enthusiastic guide during Rakka’s earliest days in Glie, and the bond they share makes Kuu’s departure all the more devastating for Rakka. With the other Haibane, Kuu shares a warm, mutual affection. She looks up to Reki and tries to imitate her, while Hikari, Kana, and Nemu treat her with fond, tolerant care. Her departure leaves a profound void in the household.

Kuu’s development arc is compact. Initially, she struggles with the frustration of being the smallest and youngest of the older group, a feeling that sometimes manifests as a mild envy of those around her. She attempts to bridge the gap by borrowing the possessions and roles of the older Haibane, actions that reflect her desire to grow up faster. However, over time she comes to accept herself and her unique position in the family. The contentment she finds allows her to achieve her Day of Flight with a quiet, joyful readiness, leaving behind only a few small mementos, such as her medicine bottle and her work apron, as parting gifts for her friends.

Her notable abilities are not supernatural beyond the common traits shared by all Haibane, such as having gray wings and a halo. Rather, her strengths lie in her effervescent energy, her uncanny ability to lift the spirits of those around her, and her instinct for forming quick bonds of friendship. She is also resourceful and curious, approaching each day as an adventure, which, combined with her accidental pratfalls, makes her a memorable and endearing presence in the quiet world of Old Home.