Jin Kariya is the primary antagonist of the anime-exclusive Bount arc. He is the leader of the Bounts, a race of immortal beings who consume human souls to survive, and he serves as the central figure driving the conflict against the Soul Reapers. Originally born with the name Eugene Currier, he adopted a Japanese-style name later in his life. As a child, Kariya had brown hair and brown eyes, but his appearance drastically changed after he first merged with his Doll, resulting in short white hair and distinctive red eyes. As an adult, he is a tall and muscular individual, often seen with a calm and confident demeanor, and bears a scar on his chin from an attack by a Hollow in his youth.

Kariya's personality is a complex mixture of intelligence, ruthlessness, and a deep-seated weariness born from centuries of existence. Initially, he appears to be a stoic and calculating leader whose goal is to gather living spirits to empower himself and his followers. However, this objective is revealed to be merely a stepping stone toward a far more ambitious and destructive plan: to invade and overthrow Soul Society. His intelligence is notable, as he possesses extensive knowledge of Soul Society, including its noble houses and the existence of the Jōkaishō, a device capable of obliterating the Seireitei. Despite his calm and philosophical exterior, he shows no genuine emotional attachment to his subordinates, whom he candidly admits to viewing as mere pawns for achieving his goals. This coldness, however, was not always the case; in his younger days, he was far more compassionate, showing great anger when other Bounts were killed and shedding tears for a fallen comrade named Cain.

The driving force behind Kariya's actions is a deep, generations-old grudge against the Soul Reapers. He was created as a result of a failed experiment in Soul Society, a mistake that the Soul Reapers later tried to erase by slaughtering the Bounts. He witnessed the massacre of his people firsthand and was forced to flee, living a life of persecution and suffering. This history of betrayal and genocide hardened his heart, transforming him from an innocent and caring individual into a determined leader seeking vengeance. His ultimate goal is not simply power, but to use the Jōkaishō to annihilate the Seireitei. However, the protagonist Ichigo Kurosaki speculates that Kariya's true, perhaps subconscious, desire is to end his own eternal existence and erase the Bounts from history entirely.

Throughout the story, Kariya's key relationships are defined by his singular mission. He was once married to a fellow Bount named Yoshino Sōma, whom he saved from suicide and convinced to live with him. He told her that their eternal life was meant for eternal bliss, but he never truly loved her, and she eventually left him after witnessing him consume a living human soul. His relationship with Gō Koga is also significant; Koga was an old friend and one of the few Bounts Kariya seemed to trust, though Kariya ultimately stabs him without hesitation when Koga questions his destructive path. He also recruited Maki Ichinose, a former Soul Reaper disillusioned with his own kind, by saving him from a Hollow. Maki became deeply loyal to Kariya, only to be killed by him when he became an obstacle. These interactions highlight how Kariya's obsession has consumed all traces of his former empathy, leaving only a pragmatic will to achieve his revenge at any cost.

In terms of development, Kariya's arc follows his transformation from a victim of circumstance into the very image of a cold and unstoppable avenger. His character is often noted for possessing depth unusual for a filler antagonist, showing moments of philosophical reflection and even engaging in mundane activities like fixing a clock or gambling, which makes him feel more human. Yet, he never waivers from his final plan, demonstrating an unwavering conviction that ultimately isolates him from everyone who once cared for him, solidifying his role as a tragic figure consumed by his own hatred.

Jin Kariya possesses a range of notable abilities that make him an extremely formidable opponent. As a Bount, he absorbs human souls to maintain his immortality and empower himself. He is also a master hand-to-hand combatant, using his centuries of experience to fight with swift and accurate movements, and has displayed immense raw strength, such as knocking Ichigo through a wall with a simple finger flick. He wields a tremendous amount of spiritual power, allowing him to fight on equal terms with captain-level Soul Reapers. His most unique ability is his Doll, named Messer, which is German for "knife". Unlike other Bounts who summon their Dolls as separate entities, Kariya has merged with his Doll, allowing him to control its power without releasing it into a distinct physical form. In its true form, Messer grants Kariya the ability to manipulate wind at incredible speeds, even generating lightning, and allows him to move at high velocities similar to a Soul Reaper's Flash Step. This fusion also caused his hair and eyes to change color, symbolizing the permanent bond between him and his power.