Asumu is a young boy who appears as a central figure in the World of Hibiki, serving as the dedicated apprentice to the master warrior known as Hibiki. His deepest aspiration is to follow in his master's footsteps and become a fully-fledged Oni, a warrior who uses specialized sound-based attacks known as Ongekido to fight monstrous creatures called Makamou. He is specifically training in the taiko drum style of this art, and he is presented as the only student of this particular school.

As a person, Asumu is defined by his earnest and devoted nature. He holds a profound respect and admiration for his master, Hibiki, who once told him that he possessed the talent to become an Oni. This encouragement gave the young apprentice a sense of purpose and direction, making him intensely loyal. This loyalty is severely tested when Hibiki suddenly and inexplicably expels him from his tutelage, yet Asumu remains determined to follow and understand his master. His primary motivation is to prove himself worthy of his master's belief and to one day achieve the status of a true Oni, carrying on the legacy of his teacher.

Within the story of Kamen Rider Decade, Asumu's role evolves from that of a hopeful apprentice to a successor. A critical turn of events occurs when Hibiki begins to lose control of his own Oni powers, gradually transforming into a rampaging creature called Gyuki. In a final act of trust, Hibiki gives his transformation device, the Henshin Onsa Onkaku, to the traveling thief Daiki Kaito, who then passes it to Asumu. By using this device, Asumu inherits the power to transform into Kamen Rider Hibiki. He is then forced to use these new abilities to confront and free his master's soul, a tragic but necessary act that completes his rite of passage. Following this, he works alongside other Oni like Akira and Todoroki to unite their previously rival schools. His journey is further complicated when his world is consumed into the greater Rider War, where he temporarily aligns himself with and against the main protagonist, Tsukasa Kadoya, before ultimately being erased from existence and later revived to aid in the final battle against a powerful enemy organization.

Asumu's key relationships are central to his development. His bond with his master, Hibiki, is the driving force behind his entire journey, from admiration to heartbreak and finally to a somber succession. He also forms a connection with Daiki Kaito, who acts as an unlikely intermediary, delivering the means for Asumu to fulfill his destiny. He fights alongside other Oni such as Akira and Todoroki, finding allies in his new role. Later, he interacts with Tsukasa Kadoya, the titular Decade, and his companions, initially blaming Tsukasa for the destruction of worlds before eventually joining him to repay the favor of having his world saved.

In terms of development, Asumu begins as a student who has not yet mastered his abilities. In this incomplete state, he is able to assume an intermediate form known as Transformed Asumu. This form is silver and muscular but lacks a proper Oni mask, and it is significantly less powerful than a full Oni. His true evolution happens when he inherits Hibiki's power. As the new Kamen Rider Hibiki, he transforms by tapping his tuning fork-like Onkaku against a solid surface, which engulfs his body in a fiery purple aura. In his Oni form, he becomes a towering figure with a secondary face-like crest on his forehead, a form stated to be his true body rather than a suit. In this state, he wields the Ongekibou Rekka drumsticks and uses the Ongekiko Kaentsuzumi buckle on his belt to perform his finishers. Asumu's abilities are further augmented when, with the help of Decade's K-Touch, he gains access to the more powerful Kamen Rider Armed Hibiki. In this armored form, he dons a samurai-like helmet and enhanced armor, significantly increasing his physical parameters and destructive power. An interesting quirk of his transformation is that while his body grows to the size of an adult Oni, his voice remains that of a young boy.