Aya Kujō, born Aya Yotsuba, is the younger sister of Tamaki Yotsuba. Separated from Tamaki during childhood due to her adoption, she was taken in by Takamasa Kujo. This arrangement settled her adoptive father's significant debts in exchange for Kujo guiding her idol career, leading to her overseas training in America. Under Kujo's influence, Aya became dedicated to his ambition of creating a new legendary idol surpassing Zero, believing his happiness depended on it.

Kujo identifies Haruka Isumi as her boyfriend, though their relationship's exact nature is unspecified. She shares a close familial bond with another of Kujo's adoptees, Tenn Kujo, whom she calls "Onii-chan". This connection indirectly involves her in the conflict between Tenn and his biological twin brother, Riku Nanase.

Her reunion with Tamaki was orchestrated by Sogo Osaka. He spread a false rumor that Tamaki had fallen ill after receiving a keychain from her, prompting Aya to rush out in immediate concern. Her reckless road crossing nearly caused an accident, leading Tsumugi Takanashi to facilitate the siblings' meeting. Tamaki had originally become an idol partly to increase his visibility on television and find her.

Her surname "Kujō" comes from her adoptive family, bearing the characters for "nine" (九, ku) and "article/paragraph" (条, jou). Her given name "Aya" (理) carries meanings related to "reason" or "truth". She continues training abroad, with her involvement in Kujo's plans and her familial ties influencing broader events.