Bos Brunnen is the sole son of the influential Brunnen family, which controls the water source serving the village of Rasenboden. As the heir to this powerful household, he is acutely aware of his social standing, which often expresses itself as an arrogant and prideful demeanor. He is frequently seen in the company of his underlings, using his position and strength to provoke the central group of friends, particularly targeting Ryza, Lent, and Tao. Despite his outward show of bravado and his noted physical strength, his true specialty lies in strategic thinking and intellectual pursuits.

The conflict that defines his early role in the story stems from a shared history. Childhood friendships once bound Bos to Ryza and her companions, but a specific incident drove them apart. During an adventure in a flooded area of the island, Ryza fell into the water. While Lent and Tao tried to pull her to safety, Bos ran to find an adult for help, knowing that a family servant was nearby. The group perceived his actions as abandoning her, leading to years of mutual resentment, rivalry, and antagonism. This deep-seated rift fuels his constant challenges and his need to prove his superiority, especially as Ryza begins to gain recognition as an alchemist and from the villagers. He is deeply distrustful of outsiders, including the travelers Empel and Lila, and he scoffs at alchemy, dismissing it as an untrustworthy or false art.

Motivated by a desire to uphold the honor of his family name and prove his own bravery, Bos often undertakes reckless excursions to the mainland, which frequently end in failure and require rescue by the very people he seeks to outdo. This reckless desire to prove himself eventually leads him to flee the village with a subordinate, an act that results in his entrapment in another world known as the Underworld. There, he is protected by a girl named Kilo Shiness, and this experience becomes the catalyst for profound change. While in the Underworld, he discovers that his family’s water source is, in fact, stealing water from that realm. This revelation, combined with his time with Kilo, forces him to reevaluate his actions. Upon returning, he reconciles with Ryza and her friends, mending the long-broken bonds of their childhood.

Following these events, Bos’s development continues. He stays behind to protect the village during the final battle against the main threat, later returning to the Underworld to destroy the harmful water source once a permanent solution is found. Driven by his intellectual strengths, he decides to leave the island to study at the Royal Capital alongside his former rival and now-friend, Tao. By the time of subsequent adventures, he has matured significantly, studying hard to become a suitable heir, though he retains his sharp tongue and straightforward manner. His abilities have grown as well; he maintains his physical prowess and has become proficient in combat, capable of fighting alongside his companions. On a personal level, his connection with Kilo Shiness remains a powerful motivation, as he continues to seek a way to reunite with her, a journey that ultimately leads to a profound resolution of his emotional arc.