Hikari emerged as a cosmic energy entity charged with orchestrating mass extinctions across galaxies. Before reaching Earth, she existed as an emotionless, formless luminous orb. During her Earth-annihilation mission, encounters with humans Takeru and Ryou triggered her fracture into two entities: Hikari embodying extinction and Akari representing survival.

She materializes as a humanoid girl with vibrant orange hair, emerald eyes, and an idealized physique, first discovered unclothed in a forest by Takeru. Her form shifts with dominant aspects - benevolent Hikari retains orange-green features while malevolent Dark Hikari manifests pink hair and crimson eyes. This duality stems from her original destructive programming clashing with emotions awakened through human bonds. Dark Hikari personifies her ruthless celestial mandate, taunting Earth’s fragility, while her gentler self arises from merging with Takeru’s compassion during embodiment.

Amnesia about her cosmic function fuels existential confusion that evolves into human affection. Displaying naive wonder toward mundane earthly wonders, she forges an inseparable bond with Takeru, who names her and becomes her emotional foundation. This connection drives her to reject annihilation duties despite Dark Hikari’s resurgence attempts, culminating in integrating both identities to spare Earth.

Her relationships reflect opposing forces: perceiving Akari as a mirrored sibling, clashing with Mari over unreciprocated feelings for Takeru, and maintaining a platonic alliance with robotic guardian Ioneos. Her evolution centers on embracing human sentiments like love and compassion, overriding programmed destruction. When Takeru sacrifices himself to halt her rampage, she resurrects him using waning powers, symbolizing her transcendence. Departing with Akari to traverse the cosmos, she vows eventual reunion with Takeru, leaving Earth’s fate delicately unresolved.