Kaito is the Prince of the Forland Kingdom and Princess Alita's older brother, eight years her senior. Initially presented as a kind and skilled swordsman, he departs Forland to broker peace with the neighboring Grandel kingdom. During his absence, Dr. Akamashi orchestrates a coup, killing the king and forcing Alita to flee. Kaito sends no further communication until his sudden, unexpected return.

He reappears transformed as the heavily armored Dark Knight, aligned with the sorceress Cecilia. This marks a stark departure from his previous benevolent nature. Demonstrating considerable swordsmanship, Kaito operates under Cecilia's influence, though the precise nature of their bond—manipulation, brainwashing, or shared purpose—remains ambiguous.

Driven by profound disillusionment with humanity's history of relentless warfare, Kaito seeks to activate Teoria, an ancient weapon of mass destruction. His goal is the world's merciful eradication, placing him in direct opposition to Alita and Falis, who fight to protect Forland.

Their conflict culminates in a confrontation. The manga concludes with Alita killing Kaito in their final battle, while the anime adaptation ends with his capture and likely imprisonment by Alita and Falis, leaving his ultimate fate unresolved.

Kaito's relationship with Alita deteriorates completely. Once fondly regarded by his sister, he becomes her primary antagonist. Alita condemns his actions as a disgrace to their royal lineage, especially his threats against Forland and its people, framing their dynamic through betrayal and familial conflict.