Akari materializes from an unexplained light in the forest, found by Ryou Ninomiya soon after Hikari appears. She initially possesses no memory of her past or purpose. Ryou names her "Akari," meaning "Bright," and brings her into his home with his younger sister Kimi. Akari develops a close family-like bond with both, though Kimi shows protectiveness over Ryou's attention.

Akari recognizes Hikari as her older sister, yet her initial reactions display instinctive fear. This stems from their shared origin as one cosmic entity embodying the cycles of extinction and rebirth. Upon reaching Earth, this entity interfaced with Ryou and Takeru's emotions, fracturing into two distinct physical forms: Hikari embodying destruction and Akari embodying survival and the desire to protect life.

Her growth centers on embracing human experiences and relationships. She exhibits childlike wonder and innocence, often consuming large amounts of food and displaying naivety about human customs. As events unfold, her protective instincts intensify, especially toward Ryou and the world. During the climax, she confronts Hikari's destructive persona, transforming her arm into an energy blade to physically oppose her sister's attempt to trigger Earth's extinction.

Akari's bond with Ryou deepens into profound mutual affection, though she defines it as familial rather than romantic. In a key moment, she gently rejects a kiss from Ryou, reaffirming their connection as family before accepting a kiss on her forehead. Her companion, the floating entity Kuon, assists her and later hibernates into a crystal fragment.

In the resolution, Akari acknowledges her true nature as part of the cosmic entity but chooses to depart Earth with Hikari. They wander the cosmos as complementary forces, though both express hope to return. This departure underscores her acceptance of both her role in universal cycles and her emotional connection to humanity.