Kio Kakazu is a 16-year-old Okinawan high school student whose ordinary life—centered on anime, video games, and sci-fi—shatters after encountering an alien named Eris. He sports brown hair, blue eyes, glasses, and stands 1.60 meters tall. Kind-hearted and driven by justice, Kio displays situational naivety, immediately accepting Eris’s alien nature and discovering his friends Manami Kinjō and Aoi Futaba are covert agents. Despite his composed exterior, he accidentally exhibits perverted tendencies, including frequent nude encounters and a hidden porn stash Eris finds and misinterprets culturally. He remains oblivious to romantic advances from multiple girls.

Formerly shy, Kio was drawn out of his shell by childhood friend Manami. He lives with his uncle, Yuichi Miyagi, who has unexplained firearms expertise. Kio’s alcohol intolerance causes him to pass out after one sip of beer during Eris’s arrival, leading to her waking half-naked in his bed.

Kio evolves from passivity to proactive heroism, risking his life to rescue captured Eris by leveraging his uncle’s resources and blackmailing the group "Beautiful Contact." His home becomes Earth’s official Catian embassy, granting diplomatic immunity to defectors Manami and Aoi. During a crisis, he uses the Catian captain’s bell to temporarily transform into a catboy, gaining feline ears and command authority to override a mothership’s security. In the climax, he dons a form-fitting power suit to battle advanced mechs, startling allies with his combat prowess.

Romantically, Eris selects Kio as her mate after his kindness during her first heat cycle, though she delays mating for diplomacy. Manami harbors long-suppressed feelings for him, while Aoi falls for his acceptance of her psychic abilities. The trio resolves their rivalry when Eris proposes sharing Kio under Catian customs, culminating in all three kissing him in the finale.