Ioneos hails from the advanced civilization Cosmo Entelecheia as a prince. His core mission is retrieving the potent cosmic energy source Tarsian from Earth. A skilled warrior, he wields shape-shifting abilities, multiple weapons, and energy manipulation for combat. Initially, he operates with calculated precision, prioritizing duty through logic over emotion, which renders him distant.

After arriving on Earth, Ioneos encounters humans—notably Takeru Takemoto—and gradually forms loyal bonds with them. This sparks a pivotal shift: he risks his safety to protect them, subordinating his original mission to their well-being. Through these interactions, his latent empathy surfaces, contrasting his earlier detachment.

As guardian to Hikari, a cosmic entity embodying extinction, Ioneos is accompanied by Kuon, a smaller being of similar origin. He initially perceives Hikari's destructive "dark" persona as her true nature, dismissing her benevolent human-influenced identity as a glitch. This conviction drives him to support her role in triggering Earth's extinction cycle, aligning with his cosmic duty.

Throughout the narrative, Ioneos grapples with loyalty to his civilization versus attachment to Earth's inhabitants. His worldview transforms when Hikari rejects destruction to spare humanity. He accepts her choice, respecting her autonomy and evolved identity. In the climax, he enters hibernation alongside Kuon, crystallizing into a dormant entity.

Ioneos embodies the conflict between duty and personal bonds. His journey culminates in recognizing truth and purpose as non-absolute, echoing his creed: "I want to see the truth, no matter how ugly it is."