Kain Blueriver is the twenty-year-old protagonist of the series, a young man who works as a professional trouble contractor, taking on cases that are too difficult or dangerous for standard authorities to handle. He stands approximately 172 centimeters tall and carries the weight of a significant legacy as the inheritor of the Swordbreaker, a legendary lost ship passed down to him by his grandmother, Alice. Kain is characterized by a hot-blooded, straightforward, and often impulsive personality. He has a notoriously short temper and is easily irritated, especially when his peers criticize his fashion sense or, more annoyingly to him, mistake him for a girl. Despite his abrasive surface, he possesses a deep-seated sense of justice and a relentless drive to fight against the entity known as Dark Star, partly motivated by a desire for revenge for his grandmother's death.

In his role as the captain of the Swordbreaker, Kain works alongside two primary companions: Canal, the holographic sentient program that serves as the ship's main computer, and Millie, a spirited young woman who joins their operations. While he often argues with both of them, particularly when Millie"s cooking goes wrong or Canal teases him, he forms a strong and unbreakable bond of trust with his crew. This loyalty is tested and proven when he discovers Millie is the granddaughter of the leader of the Nightmare军团, the very organization he is fighting against. Despite this revelation, Kain chooses to continue trusting her completely based on her actions and character rather than her lineage. He has a specific appreciation for certain foods, particularly salami and the meals Millie prepares, and holds a deep admiration for twentieth-century culture, including old films and animations.

Kain is defined by several distinctive quirks and abilities. He has a well-known obsession with collecting black capes, owning an entire wardrobe full of them, and becomes violently angry if anyone dares to insult his attire. His combat abilities are formidable; he is highly proficient in swordsmanship and wields the Light Sword, a powerful psychokinetic blade that requires immense mental strength and focus to manifest and control. Beyond his combat skills, Kain possesses naturally enhanced hearing and an almost precognitive ability to sense danger before it strikes, making him a highly effective pilot and tactician.

Throughout his journey, Kain faces overwhelming odds, dealing not only with the relentless assaults from the Nightmare军团 but also with obstruction and suppression from the Cosmic Army. He often finds himself outnumbered and outgunned, yet his natural optimism and refusal to give up allow him to push through seemingly impossible situations. Over the course of the story, Kain evolves from a young man acting largely on personal vengeance to a more mature hero who fights for the protection of his friends and the future of the universe. His development is marked by his ability to set aside prejudice and his deepening, unspoken romantic affection for Millie. In the anime adaptation, his journey concludes with a bittersweet sacrifice, as he and Canal successfully destroy Dark Star but are subsequently lost adrift in the vastness of space.