Asumu Adachi is a central figure in his story, often considered a secondary protagonist whose journey of self-discovery is as important as the battles against monsters. He is a young boy on the verge of entering high school, living alone with his mother following his parents' divorce. Before his high school entrance exams, he travels to Yakushima where he has a fateful encounter with a man named Hibiki. After witnessing Hibiki transform into an Oni to save him from a creature called a Makamou, Asumu becomes deeply fascinated and drawn into a hidden world of warriors who protect humanity.

Asumu is characterized by a kind and thoughtful, yet timid and sometimes uncertain personality. He is at a crossroads in his life, unsure of his own path and what he wants to become. This personal uncertainty is the core of his character. His motivations are not to gain power, but to find his own identity and purpose. He looks up to Hibiki not just as a hero, but as a mentor and a model for how to be a strong, capable adult. His role in the story is primarily that of an observer and a learner. He is the audience's window into the world of the Oni, experiencing their secret battles and daily lives from an outsider's perspective. He begins working part-time at the sweet shop Tachibana, which serves as a front for the Oni's support organization, further integrating him into their world.

Throughout the series, Asumu forms several key relationships. His bond with Hibiki is the most significant, a mentor-student dynamic built on mutual respect and care, where Hibiki offers quiet guidance rather than direct instruction. He is friends with Hitomi Mochida, a kind childhood friend, and becomes close with Akira Amami, a girl training to become an Oni. The arrival of Kyousuke Kiriya, a rival who aggressively seeks to become Hibiki’s disciple, creates tension and forces Asumu to confront his own feelings about becoming an Oni.

Asumu's development is the heart of his character. For much of the story, he considers following in Hibiki's footsteps. He even begins training to become an Oni, driven partly by Kiriya's competitive spirit. However, through his experiences and conversations with Hibiki about life, death, and purpose, he has a profound realization. He understands that his true calling is not to fight as an Oni, but to save lives in a different way. He ultimately leaves his apprenticeship and his part-time job to pursue a career as a doctor. In the end, he is studying medicine, having found his own path to becoming a person who can truly help others.

While Asumu never becomes an Oni in the main television series, he demonstrates potential and achieves a transformation in other media. In the theatrical film, he fully becomes Hibiki's apprentice and helps him achieve a powerful new form. In a Hyper Battle DVD, he is even able to transform into Kamen Rider Armed Hibiki himself, learning from talking Disc Animals that a normal person can become an Oni by mastering eight factors, including making sound an ally, daily training, and never giving up.