Mizuki Kusanagi is the biological daughter of Suito Kusanagi, conceived during a sexual encounter between Suito and her superior officer, "Teacher," while Suito was emotionally unstable after her friend and fellow pilot Higasawa's death. Suito initially sought an abortion, but Teacher intervened without consent, arranging artificial nurturing of the fetus, resulting in Mizuki's birth through artificial means.

Mizuki appears as a prepubescent child with long black hair and white eyes, typically dressed in dresses featuring bows or capes. Upon meeting others, such as pilot Yuichi Kannami, she initially identifies as Suito's younger sister, though others quickly correct this to reveal their true mother-daughter relationship.

The dynamic between Mizuki and Suito is strained and complex. As a genetically engineered "Kildren," Suito remains perpetually adolescent and cannot physically age, while Mizuki grows normally. Suito expresses resentment toward Mizuki, stating she hates herself when seeing her daughter and dreads Mizuki eventually outgrowing her. At times, Suito voices direct hatred toward Mizuki. Yet subtle gestures—like Suito squeezing Mizuki's hand when believing herself unobserved—hint at underlying affection. Suito also defies expectations by keeping Mizuki at the military base instead of sending her away.

Mizuki serves as a narrative symbol of the Kildren's existential tragedy. Her natural growth underscores Suito's entrapment in perpetual adolescence, highlighting the Kildren's inability to experience maturation, parenthood, or conventional futures. Her presence reinforces themes of cyclical replacement and the ethical consequences of artificial human engineering.

No significant character development or expanded background for Mizuki is detailed. Her role remains primarily thematic, illustrating the consequences of her mother's existence within the story's dystopian framework.