Sayoko Arashiyama, known as Arashi, is a spectral remnant of a World War II air raid that claimed her life as a student at Ookurayama Girls High School. Bound eternally to summer’s warmth, she materializes as a teenager frozen at sixteen, her spirit anchored by unresolved loyalty to her best friend Kaja, whom she lost amid bombing chaos. Her memories persist intact: a privileged yet rigid upbringing, a fierce legacy of defending bullied peers with a wooden practice sword and equestrian prowess, and the desperate search for Kaja that ended in her demise. This unresolved grief fuels her ghostly purpose—to traverse time, rescuing others from historical tragedies.

Arashi’s power hinges on forging bonds with the living, enabling shared journeys to critical moments in history, often targeting wartime air raids to alter fates. These expeditions demand her faded school uniform and close proximity to her partner to maintain cohesion. Strict temporal laws forbid confronting past incarnations of herself or others, under threat of erasure. Her partnership with Hajime Yasaka, a modern thirteen-year-old boy, drives her missions, though she privately harbors affection for an older, time-traveling version of Hajime she once encountered—a paradox intertwining her past and present.

Her survival relies on energy siphoned from living companions or spectral peers, forcing reliance on alliances. Deprivation gradually erodes her form. To sustain herself, she works at a retro-styled café run by Sayaka, where her bubbly charm and old-fashioned tastes clash with endearing clumsiness and bafflement toward modern gadgets. A familial taboo prevents her from acknowledging heat or cold; instead, she hums the "Warabune Tune" when overheated, triggering an unspoken rule requiring others to cool her surroundings.

Arashi’s selflessness shines in her guardianship of Yayoi and Kanako, fellow wartime spirits she shields from energy depletion. Though optimistic, she wrestles with the heartache of ephemeral bonds, dreading romantic entanglements that mortality will inevitably sever. Her dynamic with Kaja mirrors their life—a steadfast but ideologically divided friendship, as Kaja resists altering history while Arashi intervenes relentlessly. Energetic and whimsical, she dons playful café costumes and navigates absurd mishaps like body swaps, yet introspection shadows her humor, revealing a girl grappling with loss, duty, and the fragile rules sustaining her existence. Forgetfulness and rigid self-discipline add complexity, painting her as both a relentless savior and a ghost achingly human in her flaws.