Kazuma Mikura, almost always called Kazu by his friends, is a fifteen-year-old student and a central member of the Storm Rider team Kogarasumaru. He is of Eurasian descent, with ash-blonde hair and blue eyes, and is almost never seen without his signature white beanie. The origin of this habit is a childhood source of embarrassment, as he wore a hat to hide a makeshift diaper his sister put on his head after he continued to wet his bed until the age of six. Kazu lives with his older sister while his parents are overseas.

In personality, Kazu is defined by a sharp contrast between his outward insecurity and his fierce inner dedication. He is generally quiet, self-effacing, and often feels overshadowed by the natural and overwhelming talent of his best friend and team leader, Itsuki "Ikki" Minami. This lack of self-confidence frequently causes him to underperform, as he is prone to doubting his own abilities and feeling like a burden to his team. This insecurity is so pronounced that he once tried to give up his turn in an important battle, believing someone else would be more capable. However, underneath this timid exterior lies an exceptionally hard-working and loyal young man. Of all the members of Kogarasumaru, Kazu trains the hardest and most often, driven by the belief that he must make up for his perceived lack of innate talent. He is also something of a moral compass when it comes to the opposite sex, often being embarrassed by the lecherous behavior of his friends Ikki and Onigiri.

Kazu's role in the story is that of the loyal best friend and the unsung hero who must overcome his own self-doubt to realize his potential. His primary motivation is his intense loyalty to Ikki; he values his friend's pride and well-being above his own. This loyalty was demonstrated when he single-handedly fought five former gang leaders who wanted him to betray Ikki in exchange for a leadership position. Kazu's personal journey is about stepping out of Ikki's shadow. Initially a "speed-type" rider, his hidden potential is recognized by Spitfire, the reigning Flame King, who takes Kazu under his wing as his protégé and eventual successor to the powerful Flame Regalia. This mentorship places a tremendous burden on Kazu, especially after Spitfire is killed while protecting him, leaving Kazu with a deep sense of survivor's guilt and responsibility. This event, along with the subsequent theft of the Regalia, sends him into a deep depression, from which he must be pulled by his friends. Ultimately, his struggle is to move from being a sidekick to becoming a true rival and king in his own right.

His most significant relationships are with his teammates in Kogarasumaru. His friendship with Ikki is the core of his character; it is both the source of his insecurity and his greatest strength. He also forms a crucial bond with his mentor Spitfire, who sees his potential and sacrifices everything to ensure Kazu's future as the next Flame King. After Spitfire's death, he is also guided by Aeon Clock. Within the team, he is a steady, if sometimes reluctant, anchor, even serving as the interim leader when Ikki is hospitalized.

Kazu undergoes significant development throughout the series. He begins as a boy plagued by a lack of confidence and a tendency to fade into the background, earning him the derisive nickname "Emo." Through grueling training and the harsh lessons of defeat, he awakens to his true path as a rider: the Flame Road. A major turning point is a fight with Ikki, who disguises himself to challenge a depressed Kazu. During this battle, Kazu decides to finally stop trying to follow Ikki and instead stand against him as a rival. By the end of the series, he overcomes his crippling self-doubt, masters the Flame Regalia, and achieves the status of a true king, eventually defeating a powerful opponent named Zeus by burning his own soul as fuel. Despite his fearsome new power, he retains his core personality, and in the end, he is confessed to by a girl named Mikan.

Kazu is a speed-type rider, and his inherent talent for velocity is so great that he can move faster than the eye can see, becoming effectively invisible. His "Road," or special fighting style, is the Flame Road, which he inherits from Spitfire. His abilities center on generating and manipulating heat and flames, often as a byproduct of his incredible speed. His signature technique is "Time," where he uses his acceleration to strike specific nerve points on an opponent's body, stunning them and creating the sensation that time has stopped and that they are on fire. He also develops a defensive version called "Shield of Time," which uses the same principle to slow incoming attacks. Other moves include "After Burner," which propels him with jet-like bursts of flame, and "Flame Lens," which allows him to project illusions. The ultimate ability granted by the Flame Regalia is "Hole Nine Heaven's Door," a dangerous technique that opens a link to a data archive of all riders, allowing him to perfectly copy any trick or Road he witnesses.