|Floating in the sky
Kuu is the youngest of the Old Home haibane before the arrival of Rakka, not counting the Young Feathers. She is free-spirited, cheerful but also quite inquisitive as indicated by certain clues in her room (see Threads). She works at a restaurant. Kuu has something of an affinity for cats, being shown playing with them several times. Her character seems to resemble that of Niea (with whom she shares this affinity) from NieA_7, a previous project that ABe was also involved in, though unlike her alien counterpart, Kuu actually has manners. Initially thought to have gone out to the town, she takes her Day of Flight in episode 6. This deeply hurts Rakka, with whom Kuu developed a friendship cut short by her departure.
To be added later.
In addition to her cat affinity, Kuu seems to have had a special relation with flight and aeroplanes (of which are, unsurprisingly, none in Guri). She is seen several times to be running off with her arms stretched out, imitating an airplane. She also tells Rakka that even though their wings are quite unusable, she is certain that they will be able to fly someday. This foreshadows her impending Flight. (Also note that Kuu is wearing a cap by the time she tells this, possibly to hide the halo's flickering.) If Kuu indeed had a trial during her time in Guri, it was that of overcoming her childishly competitive and attention-seeking behaviour. This ironically led to her early passing over the Wall.
Flight attendant Clarabelle "CB" Lansing, while working for Aloha Airlines, was ejected from Aloha Flight 243 by being caught in the incipient failure of the aircraft fuselage 24000 feet above the Hawaiian islands. It is a near certainty that she lost consciousness before reaching the surface, and that the impact event with the fuselage would probably have preempted the sense of falling, which is what distinguishes Kuu's cocoon dream from Rakka's. While there are other airliner ejections such as these, Lansing was a flight attendant for 37 years, explaining Kuu's apparent affinity for aircraft, and far from retirement, explaining Kuu's need to "fill her cup" according to episode 5 (09:43).