Kozue Satō, Kū Shiratori's energetic and friendly best friend, sports short black hair barely reaching her neck and almost always wears bulky red goggles atop her head, despite their lack of apparent function. Her brown eyes are often seen peering from beneath them while she's in her school uniform. Kozue exhibits deep concern for Kū's well-being, especially during Kū's periods of emptiness or depression. Noticing Kū's sadness over lacking a dance partner for the school festival, Kozue offers herself as a partner, feeling profound relief and joy when Kū accepts, viewing it as a significant emotional connection.

Her energy is drained by the Absolute Angel Tarlotte during a school attack. After Kū disappears following Kyoshiro Ayanokoji's arrival, Kozue persists in trying to contact her, remaining worried about Kū's whereabouts; Kū remains unaware of these efforts, mistakenly believing Kozue has moved on. Kozue lives without either parent present. Her name "Kozue" means "treetop" or "tree branches," while her surname "Satō" combines the characters for "help/aid" and "wisteria."

Kozue displays a strong emotional attachment to Kū. She reacts with visible happiness upon learning Kū cannot pursue romantic feelings for Kyoshiro due to being an Absolute Angel. When Kū expresses contentment with their ordinary days together, Kozue interprets this as a love confession and openly states her desire for Kū to be her partner; she blushes after Kū accepts the arrangement, regardless of its non-romantic context. Kozue also experiences direct interaction with Tarlotte, who kisses her during the energy-draining incident.