Kareha Banouin[]
Kareha is Mizuha's older sister, with both of them having arrived at the Neighboring World at the same time. Their relationship prior to becoming Quasi-Spirits was quite close, with Kareha often protecting Mizuha from bullies.
Mizuha witnesses Kareha start to become an Empty after seeing Retsumi Jugasaki sacrifice herself to an attempt on Kareha's life. Kareha eventually dies before the transformation completely takes hold, though this doesn't affect Mizuha because of her close relationships with other Quasi-Spirits.
Rinemu Kirari[]
As her senpai in the idol industry, Mizuha greatly looks up to Rinemu as her inspiration to sing. After Rinemu lost her ability to sing, Mizuha took over her job as Dominion of Yesod. However, Mizuha would use her newfound authority to emphatically refuse Rinemu's requests to leave for another region. This was done out of hope that Rinemu would one day regain her voice and retake the position. Upon hearing rumors that Rinemu was dead, Mizuha subsequently lost her inspiration for living and began the transformation into an Empty just as she was about to perform live.
The transformation was reversed when Rinemu returned. She proceeded to upstage Mizuha with her own performance alongside Kurumi Tokisaki and Tsuan. After their performance, Mizuha cried tears of joy at Rinemu's comeback, and eventually came to understand Rinemu's ambitions as the reason why she chose not to reclaim her Dominion position.
Mizuha eventually developed feelings for Rinemu, enough to follow her out of the Neighboring World when the gate to the real world opened.