Kenji Sadamaki maintains the unassuming facade of a part-time convenience store clerk, an ordinary civilian to the outside world. Beneath this cover, he operates secretly as Divine Swordsman Blader, an unsponsored superhero defending Tokyo against threats like the evil organization Agastia. Juggling his job with unpredictable, unpaid heroics strains his finances severely, pushing him toward abandoning his duties due to public obscurity and economic hardship. However, an offhand remark from Tōka Kuroitsu, a regular customer oblivious to his secret identity, unexpectedly reignites his determination to persevere in his fight for justice.

Complicating his battles is the hidden truth that Kenji's older brother, Hajime Sadamaki, unknowingly heads Agastia's Monster Development Department, creating the very adversaries Blader faces. Neither brother recognizes their familial bond through their opposing roles, adding layers to their indirect conflict. Kenji's civilian life is further tangled by his developing crush on Kuroitsu, who frequents his store for lunch, unaware he is both her target and admirer; she actively engineers monsters designed to defeat Blader.

During his confrontations, Kenji discovers unexpected new abilities and forms, revealing unforeseen growth in his heroic potential. His reactions to suggestive encounters, particularly involving Kuroitsu in compromising field situations, often result in comedic incapacitation, like fainting from nosebleeds. Physically, Kenji stands 178 cm tall, with black hair cut to ear length and blue eyes, typically dressed in casual jeans and hooded attire.