Baishin is a Shinigami who appears exclusively in the Bleach: The Sealed Sword Frenzy OVA. His history begins roughly four hundred years before the main storyline, when he discovered a way to fuse his own body with his Zanpakutō in a misguided attempt to further his swordsmanship. This fusion, which he could not reverse, drove him into an uncontrollable rampage. During this event, he was responsible for the deaths of sixty-three seated officers, some of whom were lieutenants. After this massacre, he escaped to the human world. In response, Soul Society dispatched six Captains to capture him. The mission resulted in the deaths of two of those Captains, while the remaining four were able to successfully seal Baishin away.

In terms of appearance, Baishin is a middle-aged man with a beard and distinctive blades growing out of his shoulders. He wears samurai-like armor over his standard Shinigami hakama. His outward behavior is that of a violent and rampaging killer, but this is largely a consequence of his forced fusion with his own blade. Deep down, his primary motivation is to separate himself from his Zanpakutō at any cost. To achieve this goal, he seeks out powerful individuals in order to absorb their spiritual energy, or Reiryoku. He intends to use this accumulated power to trigger a massive explosion, one strong enough to split him from his sword, even if the process results in his own death.

Within the story of the OVA, Baishin serves as the primary antagonist. He manages to break free from the statue in which he was sealed, killing the monks guarding it. Upon his release, he is confronted by Ichigo Kurosaki, Renji Abarai, and Rukia Kuchiki. He proves to be a formidable foe, easily shrugging off their attacks and demonstrating his ability to absorb Ichigo's spiritual energy. He also displays the power to summon Hollows through small, garganta-like portals. During his final confrontation, after absorbing enough of Ichigo’s power, Baishin is able to unleash his Bankai, which transforms his body into a living mass of blades. In the end, he is defeated and killed by Ichigo, who cuts him in half. Baishin dies happily, as this act finally achieves his goal of separating his soul from his Zanpakutō, leaving only the sword behind.

Key relationships for Baishin are primarily adversarial. He has a direct history with the Gotei 13, having killed several of its Captains and lieutenants in the past. In the present, he directly battles Ichigo Kurosaki, Renji Abarai, Rukia Kuchiki, and Toshiro Hitsugaya, viewing them primarily as sources of spiritual energy to absorb. There is no development for Baishin beyond the events of the OVA, as his story concludes with his death and the fulfillment of his wish to be separated from his Zanpakutō.

Baishin possesses a number of notable abilities derived from his fused state. As a baseline, even before his transformation, he was a highly skilled swordsman and expert in Shunpo, the flash steps technique. His fusion grants him immense durability, allowing him to block attacks from captain-class opponents like Hyōrinmaru with his bare hands and remain unharmed by high-level Kido spells. His most prominent power is his ability to transform parts of his arms into blades, which he can then launch as projectiles at his enemies. He can also absorb the spiritual energy of others through direct contact. When he gathers sufficient power, he can activate his Bankai, a form that covers his entire body in hundreds of sword blades that serve as both armor and a source of countless additional projectiles.