Kaina hails from a small, isolated village on the Canopy—an organic membrane suspended by colossal Orbital Spire Trees above the planet. His community, mostly elderly, faces extinction as the trees decay, believing themselves humanity's last survivors. Orphaned young when his mother died in childbirth and his father on a hunt, he grew amid scarcity, mastering navigation and hunting across the treacherous Canopy with ropes and spears. Yet, he harbors unwavering curiosity about the world below, especially the Snow Sea.

His sheltered life fractures upon rescuing Ririha, an Atland princess who arrives on a Floater. Learning of her mission to find the mythical Sage and save her drought-stricken land from Valghian invaders, the elders task Kaina with guiding her to the Snow Sea. They provide salvaged climbing gear and a tool mistaken for a bark-cutter—later revealed as a heat-sensitive laser device. Accepting this mission means abandoning his home forever.

During their descent, Kaina discovers the surface’s water shortages, Valghia’s threat, and the Sage legend. He shares sightings of an unearthly light entity, hinting at ecological ties. Witnessing a neighboring Orbital Spire Tree collapse underscores the planet’s decay. Reaching the Snow Sea, Valghian soldiers ambush them, exposing Kaina to warfare; his inexperience breeds discomfort with violence and sacrificial ideologies, though he prioritizes practical escape.

Forced underwater with Ririha’s brother Yaona, Kaina adapts using oxygen-fruits and explores sunken ruins. He instinctively deciphers Kanji-like symbols in tree roots, suggesting ties to forgotten technology. His resourcefulness surfaces during Ririha’s rescue from Valghia: he improvises wildlife transport and deploys the laser tool, despite steering challenges. After Ririha aids their escape, he navigates Valghia’s mobile fortress, encountering its advanced tech like bionic armor. His choices consistently prioritize saving lives and seeking solutions, forging agency distinct from surface conflicts.