Mizuki Hayama, called Miki in childhood, is a middle school student at Otowa Academy. At age five, she survived a traumatic incident where her mother deliberately crashed their car off a cliff into the ocean, killing all passengers except Mizuki. Placed in a church orphanage, caregivers Yuuko Amamiya and Yuu Himura helped her process the tragedy without lasting emotional damage, though a residual fear of the ocean and recurring nightmares of sinking linger.

Cheerful and forthright, Mizuki speaks candidly about private topics. She devours shōjo manga, particularly works by Hiro Hirono—unaware the author is a local high school student’s alias. Her vigor extends to the basketball team, where she idolizes senior Kei Shindou as a role model. While she intermittently daydreams about romance with Kei, these fantasies remain unreciprocated.

During a winter trip to Otowa, Australia, Mizuki encounters neighbor Kuze Shuichi through her aunt Sumire Asou. Kuze fails to recognize her, but Mizuki recalls their childhood meeting when he gifted her a blue ribbon she still wears. After discovering Kuze’s terminal illness from Chihiro Shindou, Mizuki resolves to breach his isolation despite his prognosis. Initially rejected due to Kuze’s wish to shield her from grief, her relentless positivity gradually draws him into companionship. Their connection crystallizes at a Christmas concert, emblematic of their shared resolve to value fleeting moments.

Born December 22, Mizuki is 16 at the story’s outset and 17 by its close. Earlier timelines portray her as a 15-year-old sustaining Otowa Academy ties through Kei and peers. Her arc highlights resilience in facing loss and an unyielding dedication to uplifting others, forged by her orphanage experiences and Yuuko and Yuu’s mentorship.