Mercenary is the male protagonist of the story, a member of a race known as Beastfallen. He was born with the appearance of a large, imposing white tiger, walking upright and wielding a sword, a form that is the result of an ancient curse or magical affliction. His very name is not a given one but a title that reflects his profession, as he works as a swordsman for hire, and it is the only identifier he uses.

His entire existence has been shaped by the prejudice and fear he faces from humans, who see Beastfallen as dangerous monsters created by witches. This constant persecution has made him deeply cynical, pragmatic, and initially very distrustful of others. He often comes across as brusque or even like a jerk, but this is a defensive mechanism forged from years of being hunted, betrayed, and treated as a valuable but despised commodity. Beneath his rough exterior, however, he possesses a strong sense of honor and a surprisingly kind heart. His primary motivation, the driving force behind all his actions, is a deep and simple wish to become a normal human being so he can live a quiet, peaceful life free from persecution, perhaps even as a tavern owner.

The course of his life changes when he is hired to hunt witches but is instead rescued by a young and powerful witch named Zero. He enters into a contract with her: he will serve as her bodyguard and protect her on her journey to find a dangerous magical tome known as the Grimoire of Zero, and in exchange, she promises to use her magic to grant his wish and turn him into a human. This arrangement places him at the center of a conflict involving powerful sorcerers and the future of magic in the kingdom.

His most significant relationship is, of course, with Zero. Initially, he sees her as a nuisance and a means to an end, tolerating her quirks, such as her habit of using his furry body as a pillow. Over time, their contract evolves into a genuine, if unspoken, bond of trust and companionship. He becomes fiercely protective of her, not just because of their deal, but because he comes to value her as a person. He also forms a paternal bond with a young witch named Albus, who joins their group, often acting as a reluctant but responsible caretaker for the two powerful but socially naive magic users.

Mercenary undergoes a significant transformation that redefines his core desire. When Zero, in a desperate attempt to keep him safe, forcibly transforms him into a human, he finds the experience to be a cage rather than a liberation. As a human, he is powerless and unable to protect those he cares about, which he finds unbearable. He eventually comes to accept his Beastfallen form, realizing that his strength and identity are tied to it. He chooses to remain as a white tiger, not as a cursed monster, but as a noble warrior who has found a place where he belongs. In the end, his dream of a quiet life is realized in a different way. Years later, he helps rebuild his home village, turning it into a refuge where humans, witches, and Beastfallen can coexist, and finally opens the tavern he always wanted.

In terms of abilities, Mercenary is a highly skilled and formidable warrior. His Beastfallen body grants him immense physical strength, speed, and durability, making him a powerful front-line fighter. He is exceptionally proficient with a sword, and his combat instincts and years of experience as a mercenary allow him to hold his own against not only ordinary soldiers but also magic users. Despite his gruff demeanor, he is also notably resourceful and a surprisingly good cook, a skill that hints at his domestic aspirations.