Miharu Mashiki is a first-year high school student bound by a lifelong connection to Haruki Mishima, having shared the same schools since kindergarten. Her family history includes a deceased twin sister, Misora, who succumbed in childhood to the same hereditary heart condition that afflicts Miharu. This illness leaves her with frail health, demanding constant medical care and periodic hospital stays.

She unexpectedly reunites with Haruki at a convenience store near their high school, reigniting their childhood bond. Her feelings for Haruki deepen organically through their renewed interactions, yet her recurring health crises repeatedly disrupt their connection. Frequent hospital visits, especially intense during the second semester and winter, force extended absences from school. Haruki visits her bedside daily throughout these ordeals, a testament to their growing mutual affection and concern.

Her personality radiates kindness and a supportive spirit; she readily assists friends whenever possible. This concern extends broadly, evident when she and Haruki consciously pause their budding romance to focus on aiding their friends, Towa Honda and Mami Mihashi, navigate their own relationship struggles.

Her health remains a persistent battle. A significant hospitalization in February forces her into an extended absence from school. This ongoing condition fundamentally shapes her narrative, impacting both her daily existence and the evolving dynamics of her relationship with Haruki.