Kantarō Hoshi is the son of a human scientist and Maya, an alien woman from the destroyed planet Katharos, making him a hybrid of both species. His mother came to Earth as a refugee and later died from exposure to Blaster radiation while constructing a protective robot, leaving Kantarō to be raised by his father alone.

He possesses distinct Katharosian traits, most notably crystal-blue eyes known as Innocent Blue Eyes, which signify inherent heroism. His surname, meaning "star" in Japanese, reinforces his celestial origins.

Kantarō is hot-blooded and serious, driven by a powerful sense of responsibility and a deep love for humanity that makes him an All-Loving Hero, willing to sacrifice himself for Earth’s safety. His mother’s death remains a painful trigger; he reacts with fierce anger toward Blasters who harm innocents, a tendency his father and the robot Ganger often warn him about.

Though occasionally bratty and overconfident, he remains open to guidance, especially from Ganger, who serves as an older brother figure. Still in elementary school, Kantarō carries the weight of being a Kid Hero who defends the world from alien invasion. A skilled driver, he operates a unique hot pink motorbike built by his father, using it to perform stunts and initiate transformations.

His closest relationship is with his classmate and best friend, Rie Hayakawa. He displays an Always Save the Girl attitude toward her, fiercely protective whenever she is in danger—a vulnerability the Blasters exploit by using her as bait. Initially, Rie saw him as a weakling and Butt-Monkey, mocking his clumsiness in sports and initial fear during saucer attacks. Her perception shifts when she learns he pilots Astroganger.

Kantarō merges with the sentient robot Astroganger, formed from a special "living metal" his mother brought to Earth. He does not merely control the robot but fuses with it, uniting its strength with his will and intelligence. Their partnership embodies teamwork, as they must act as one to achieve peace.

His journey is a Coming of Age Story. He evolves from a grief-stricken, angry boy into someone more mature, learning to understand the Blasters rather than simply hate them, eventually seeing them as equals. This growth is guided by his father, a Reasonable Authority Figure who advocates virtue over rage, and by Ganger, his stoic mentor.

The story ends on a Bittersweet Ending. Kantarō defeats the Blasters for good, but victory requires Ganger’s Heroic Sacrifice, using his last power to destroy the final threat and create an escape shuttle for Kantarō and Rie. Kantarō is left to witness the death of his mechanical guardian and friend.