Agatsuma Soubi is a central character who serves as a Fighter, a being designed for combat using words as weapons. He is introduced as a twenty-one-year-old university student majoring in traditional Japanese painting, possessing a striking appearance with strawberry blonde or silver hair and purple eyes, standing notably tall at 192 centimeters. His backstory is marked by profound loss and manipulation. Orphaned at the age of six, Soubi was taken in and trained by a man named Ritsu Minami at an institution known as Seven Moons Academy. This training was severe and psychologically damaging, designed to create what is known as a blank Fighter—one without a destined partner—and to instill absolute obedience to a master. Ritsu's methods included physical punishment and emotional cruelty, such as taking a young Soubi to his parents' grave and telling him to climb inside, which instilled a deep-seated fear of graves and the dark. He was also subjected to the removal of his cat-like ears at a young age, a trait common in the series' world, under the rationale that it would make him a more effective Fighter.

Soubi's personality is a complex tapestry woven from his traumatic upbringing. Outwardly, he often appears calm, mysterious, and somewhat detached, possessing a high tolerance for pain and an extraordinary ability to hide his true emotions. He is known as the perfect Fighter because of his absolute, almost mechanical, obedience to his master's commands. This obedience is not merely a trait but a core part of his identity, having been groomed to see himself as an object or a tool to be used by the person who holds his name. Consequently, he struggles to act on his own initiative and often insists that he needs orders to follow, expressing discomfort with having to make independent choices. Despite this cold exterior, he has a gentle and caring side, which emerges most clearly in his interactions with those he grows close to. He is also known to be somewhat childish in managing his own emotions, sometimes resorting to sullen withdrawal or physical violence when upset. His favorite flower is the Casablanca lily, he enjoys relaxing music, and he is a heavy smoker.

Soubi's primary motivation is rooted in his identity as a Fighter. Initially, his entire purpose is defined by his former master, Seimei Aoyagi, who carved his true name, Beloved, onto Soubi's neck, thereby claiming ownership. After Seimei's apparent death, Soubi is ordered to transfer his allegiance to Seimei's younger brother, a twelve-year-old named Ritsuka Aoyagi, who bears the name Loveless. This command sets the central trajectory of his role in the story: to serve as Ritsuka's protector and partner in spell battles, while assisting the boy in uncovering the truth behind his brother's death and confronting the mysterious organization known as Seven Moons. His actions are caught between the lingering, absolute commands of Seimei and the new, evolving relationship with Ritsuka.

His key relationships are the driving force of his character development. The bond with his former teacher, Ritsu, is one of master and slave, defined by abuse and control, which left deep psychological scars and shaped his understanding of love and obedience as something tied to power and possession. The relationship with Seimei is similarly complex and unhealthy; Seimei treated Soubi as a belonging, teaching him that love is inconstant but control is absolute. Even after Seimei's death, his commands hold power over Soubi, creating a constant internal conflict. The most significant relationship is with Ritsuka. Initially, Soubi's care for the boy may stem from Seimei's orders, but over time, it develops into a genuine and deep emotional attachment. He becomes fiercely protective and possessive of Ritsuka, showing a level of vulnerability and affection that contradicts his conditioned role as an unfeeling tool. Ritsuka begins to see Soubi not as a thing to be commanded, but as a person, which slowly encourages Soubi to confront his own suppressed feelings and desires. He also forms other connections, such as with his college friend Kio Kaidou, who provides a glimpse into a normal life, and with the Zero pair, Natsuo and Youji, whom he takes into his home after defeating them.

Throughout the narrative, Soubi undergoes significant development. He transitions from a character who unquestioningly follows orders and believes himself to be easily thrown away and unloved, to someone who begins to understand genuine affection and attachment. He starts to defy his programming, risking Seimei's potential wrath to protect Ritsuka. His greatest struggle is reconciling his status as Seimei's creation, Beloved, with his growing, authentic feelings for Ritsuka, the Loveless. The revelation that Seimei is still alive and has replaced him with a new Fighter devastates him, confirming his deepest fear of being discarded, yet it also forces him to confront the possibility of choosing his own path rather than merely being an object passed between masters.

As a Fighter, Soubi possesses notable abilities centered on word-based spell battles. He is highly skilled at finding loopholes and using an opponent's own language against them, a reflection of his strategic mind. His spells, which are triggered by spoken words, include the ability to command fire, sleep, and ice, among others, and he favors light-based attacks. Because he is a blank Fighter without a destined name, his power when paired with Ritsuka is said to be halved, yet his combat prowess and high pain tolerance make him a formidable opponent. The name Beloved carved into his neck is a mark of ownership that bleeds during battle when he is fighting for Ritsuka, a physical manifestation of his fractured loyalties. His appearance in a spell battle is often heralded by the presence of blue butterflies, a symbol that follows him and represents the recurring themes of love and betrayal in his life.