Kazuma is the central protagonist of the film, a young mercenary residing in the harsh and secluded environment known as the Lost Ground. This area was created years ago by a strange environmental phenomenon that caused the land to split and protrude upwards, leaving those who live there to fend for themselves in a difficult and unforgiving landscape. To survive, Kazuma takes on any work he can find, a necessity that has shaped his pragmatic and streetwise nature.

Personality-wise, Kazuma is known for being reckless and headstrong, a temperament that often lands him in dangerous situations. He is a powerful fighter who approaches life and conflict with a direct and uncomplicated attitude, relying on his instincts and formidable strength. Despite his tough exterior, he cares deeply for his close friends and demonstrates a loyalty that goes beyond mere survival.

Kazuma is one of the gifted individuals in the Lost Ground known as Native Alter Users, possessing the rare ability to manifest an Alter. His specific power allows him to coat his right arm and torso with a metallic alloy, transforming his body into a formidable weapon. This ability grants him incredible offensive and defensive capabilities in battle, making him a dangerous opponent. He is classified as a powerful Class A Alter user and is known by the epithet Shell Bullet.

His role in the story is primarily driven by his encounter with the organization known as HOLY, an order whose mission is to suppress and capture Native Alter Users. This brings him into direct conflict with Ryuho, one of the elite members of HOLY, sparking an epic and central rivalry that fuels much of the narrative. This conflict represents more than just a personal fight; it places Kazuma in the middle of a larger clash of ideologies between the independent Alters of the Lost Ground and the controlling authority of the Holy Land.