Gai is the central protagonist of this story, a fourteen-year-old boy who becomes the new wielder of the Rekka, or Wildfire, armor. His most immediate and defining background is that he did not begin this journey as a hero. He started as a member of the Ten Braves, a group of elite warriors serving the demon emperor Ramaga. For a time, he was an antagonist, a defector from decadence who fought on the side of evil before his reluctant defection set him on the path to becoming one of the Samurai Troopers. This past sets him apart from the traditional expectations of a hero, marking him as an anti-hero in the beginning.

Gai possesses a personality that is wild, extremely energetic, and prone to impulsive behavior. He is frequently seen with an appetite that borders on the gluttonous, often consuming large quantities of food, including an entire collection of oddly flavored instant yakisoba that belongs to his teammate, Ryusei. His taste is bizarre, as he seems to genuinely enjoy unusual and even disgusting flavors that would repel most people. When he fights, he often does so with a gleeful and almost cruel enthusiasm, a remnant of his time as a demon warrior. He is known to sing along to Japanese pop music hits from past decades as he engages in combat, a habit derived from a CD titled Demon Ultimate Collection 100 that he keeps on his person. Despite this chaotic and aggressive exterior, there are strong indications that Gai is highly intelligent and perceptive. He demonstrates a capacity for positive manipulation, such as deliberately provoking other troopers like Musashi to help them unlock their latent powers, suggesting a tactical mind forged in the militaristic culture of the demon world.

Gai's primary motivation is not simply to defeat evil but to navigate his own complex redemption and find a new purpose. Having defected from the Ten Braves, he is forced to prove his loyalty while grappling with his past actions. He is initially brought into the custody of the Demon Subjugation Troopers, the government organization tasked with fighting Ramaga, before ultimately joining their cause. His role in the story is therefore multi-faceted: he is the catalyst for much of the action, the wild card whose past connections to the enemy provide crucial insights, and the successor to the legacy of the original red-armored trooper, Ryo Sanada. His journey is one of transformation from a former enemy into a true hero and the heart of a new generation of warriors.

His relationships with the other characters are central to his development. He has a notable rivalry and scuffles with Kaito, another of the new troopers. He also frequently provokes Musashi, a fellow blue-armored trooper, to push him beyond his limits. Perhaps his most significant relationship is with the original Samurai Trooper Ryo, whom Gai calls father, indicating a direct lineage and explaining the weight of expectation on his shoulders. Over the course of the story, Gai is forced to confront the dark reality of his former allies, such as when he saves Shion from the control of the Sword of Domination. The development of Gai is a tragic and redemptive arc that culminates in his apparent death, as he becomes a statue. Ultimately, he is brought back to life, fully embracing his role as one of the titular Samurai Troopers.

In terms of abilities, as the wearer of the Rekka armor, Gai's power is intrinsically tied to fire and scorching heat. His battle cry, Now I'm fired up, reflects the nature of his power, which is fueled by his spirit and aggression. However, his strength is not without vulnerability. In a significant turn of events, he is shot by a magical bullet from one of his supposed allies, a betrayal that renders him powerless and brings him down to the level of a normal human being for a time. This loss of power becomes a crucial part of his journey, stripping him of his identity as a warrior and forcing him to rely on other qualities to prove his worth.