In the world of Air Gear: Kuro no Hane to Nemuri no Mori -Break on the Sky-, Kiriku is a character of immense significance, recognized as the former Gem King of the first Sleeping Forest. He is a Gravity Child, a being with extraordinary powers, and is the twin brother of Simca. From his earliest days among the Gravity Children, Kiriku was considered the most gifted of his entire generation, displaying a level of talent that surpassed his peers, including his childhood friend, Sora Takeuchi. His deep care for others was evident even then, as he risked his life to save other Gravity Children who were slated to be eliminated.

Kiriku's personality is defined by a profound sense of sincerity and an inherent goodness that often borders on naivety. He is not driven by a desire for power or dominance over others; rather, his motivations are rooted in protection and a deep sense of responsibility. This is most clearly seen in his pivotal role in the story's central conflict. When Sora revealed sinister intentions, Kiriku made the difficult decision to rebel against his old friend. He successfully stole the powerful Wind Regalia from Sora and took drastic measures to cripple him, hoping to prevent Sora from unleashing the Sky Regalia, an event Kiriku believed would plunge the world into chaos. This act, though appearing antagonistic to some, was a calculated and heavy-hearted decision to maintain balance and prevent a greater catastrophe, as he grappled with the moral weight of potentially being too narrow-minded in his caution. Following this, he lost his own Regalia to Nike and stepped back from the forefront of battles, preferring to orchestrate events from behind the scenes and evaluate the potential of new riders, such as Itsuki Minami, using his own unique and often critical hundred-point evaluation system.

Kiriku's key relationships define his actions and inner turmoil. His bond with Sora Takeuchi is central, transitioning from close friendship to a tragic opposition, all while Kiriku still holds a lingering sadness for the person Sora used to be. He also harbors a one-sided crush on Rika Noyamano, a vulnerability that has been known to affect him. Despite his serious nature, he has earned the goodwill of even the most intimidating riders, such as Kaito Wanijima, who recognize his justifications and feelings as more genuine than most.

Throughout the story, Kiriku demonstrates notable development. After suffering injuries that prevented him from riding Air Treks (ATs), he is later seen undergoing intense training, subjecting his body to extreme pressure to become as hard as diamond in preparation for a final confrontation with the resurrected Sora. His abilities are exceptional and varied. Kiriku follows the Gaia Road, a path that grants him the incredible power to memorize and engrave any riding script he witnesses, regardless of its complexity or speed. He is considered the strongest character in terms of raw combat power and intelligence when individual abilities are compared, and he excels in team battles by calculating all of his allies' strengths and weaknesses to orchestrate near-precognitive strategies. Even when unable to fight directly, his tactical genius allows him to lead teams to victory through remote communication alone.