Bokka Serenade is the main protagonist of the story. He begins as an unhappy and restless high school student who feels trapped by the mundane realities of his daily life. Bokka performs poorly in his classes and is openly disliked by his teachers, finding no inspiration in the conventional path laid out for him. Instead, he possesses a brave, determined heart full of a desire for adventure and a strong sense of justice. He is a rebellious member of his school's archery club, a detail that is significant because the monsters that secretly rule the world actively discourage the development of such combat skills.

His core motivation is a deep longing to become a Melos Warrior, legendary figures who once fought against the monsters that won a great war against humanity in the 20th century. While most people in his world have come to believe these monsters are a myth, Bokka feels the injustice of their hidden rule and dreams of fighting against it. This dream is fueled by the stories told to him by an old man named Tsunagi, a loiterer near his school whom others warn him to stay away from. Bokka instead befriends Tsunagi and works part-time at his auto shop, unaware that the old man is actually a leading scientist in aibanetic technology and an ally to the warriors.

Bokka's life is irrevocably changed when he witnesses a battle between a warrior named Kurofune, whom a young girl named Sayoko is searching for, and a monster. This event awakens the latent potential within Bokka to become a warrior himself. He drops out of school, leaving behind his family, to pursue his new path. When Kurofune continues on his journey, Bokka and Sayoko decide to follow him, setting the stage for the series' central quest. As he travels, Bokka sees firsthand the damage the monsters have inflicted on the world and encounters humans who have chosen to pledge allegiance to them, further hardening his resolve.

His key relationships are central to his development. Tsunagi acts as a mentor and benefactor, ultimately granting Bokka his own Aibar machine, a mechanical steed named Elan Vital. Sayoko becomes his constant traveling companion, and their relationship deepens into a romantic one as they face challenges together. Bokka also looks up to Kurofune, the experienced warrior whose example inspired him to begin his journey.

Throughout his journey, Bokka develops significantly from a disaffected student into a capable Melos Warrior. He learns to master his abilities, which are tied to a Melos tattoo, or "stigma," that appears on his left arm. His primary weapon is a bow, which he used in the archery club. After his original bow is broken in battle, he replaces it with the handlebars of his Ivermachine, Elan Vital, which transform into a new, more powerful bow complete with a built-in heads-up display. His signature technique is "Flash," a command he uses to fire his Melos arrows. He draws an arrow across his left arm, allowing it to pass over his Melos tattoo to enhance its power before releasing it at his target. He also learns a skill known as "Narration" or "Meigen," specifically "Serenade's Heart," which involves plucking the bowstring to produce a sound that can repel weaker monsters. In more advanced battles, he can achieve a state of perfect synchronization with his Ivermachine known as "Bio Concerto," which greatly increases his accuracy and attacking power.