Baishin

is a Shinigami who merged with his Zanpakutō and subsequently went on a killing rampage.

Appearance
Baishin's appearance is that of a middle aged man with a beard and blades growing out of his shoulders. He can transform parts of his arms into blades that can then launch at his enemies. He also wears a samurai-like armor over his Shinigami hakama.

History
400 years prior to Ichigo gaining Rukia's power, Baishin found a way to merge with his Zanpakutō, as a result he went on a rampage. He is responsible for the death of 63 seated officers, some of were lieutenants, and proceeded to escape to the Human World. A total of six Captains were sent to capture him, two of which lost their lives while the remainder were able to imprison him.

The Sealed Sword Frenzy
Baishin eventually broke free from the statue where he was sealed, killing everyone in the area while doing so. He hunted down those with powerful Reiatsu, and Baishin found it in Ichigo. He confronts Renji Abarai and Ichigo Kurosaki. Renji shoots him but with it has no effect and gets stabbed with a blade. Ichigo transforms into a Shinigami and begins to fights Baishin, who calls himself a blade. As Ichigo attacks, Baishin grabs him and takes some of Ichigo’s Reiatsu. Renji and Rukia attack but are too weak to hurt him. Just at that moment, Tōshirō Hitsugaya shows up and scares away Baishin. Before Baishin leaves, he unleashes a bunch of Hollows.

Traced to the site where he was once sealed, Ichigo releases his Reiatsu to track down Baishin. Baishin once again fights Ichigo, this time with Rukia and Renji at Ichigo's side. However, by sucking out Ichigo's Reiatsu, Baishin was able to perform his final release. Baishin summons his Bankai, which transforms his skin into metal, becoming a living being of swords. He is eventually killed by Ichigo using his own Bankai. After Baishin dies, Ichigo reveals that Baishin had merged with his Zanpakutō. Yoruichi thinks that Baishin tried to use Ichigo's Reiatsu to return himself back to normal, separating his body and his Zanpakutō, even if he perished. The only thing left of Baishin is the Zanpakutō he once merged with.

Powers and Abilities
Unnamed Garganta Variant: Garganta is how Hollows and Arrancar move to and from Hueco Mundo. It literally tears open the dimensional fabric separating the worlds, revealing a tunnel of whirling, torrential energy that must be focused and solidified to create a discernible pathway. Baishin used a variant of this during the OVA after draining a hefty amount of Ichigo's Reiryoku, disappearing through a portal as Hollows began to appear in their own portals.

Master Swordsman: Baishin technically is a sword. Before permanently fusing with his Zanpakutō, Baishin was a highly skilled Shinigami. Desiring to become more powerful, he fused with his Zanpakutō as it was seemingly the only way to gain more power. He cut down Lieutenants and any seated officer that got in his way, killing a total of 63 Shinigami. After escaping to the Human World, he managed to kill two captains before he was sealed.

Shunpo Expert: Baishin was able to easily outrun Ichigo using Shunpo, subsequently pinning him down.

Enhanced Durability: Baishin was unfazed by a level 31 Destruction Spell, unscathed by Zabimaru's Higa Zekkō, and was even able to deflect Hitsugaya's Hyōrinmaru with little to no effort. He also blocked Zangetsu by the hand without effort or injury. This durability only increased on a scale of 5-10 folds after his Bankai was released.

Immense Spiritual Power: Baishin also has an immense amount of spiritual energy. After he absorbed some of Ichigo's vast spiritual energy, his spiritual power became more immense than before. It is red in color.

Zanpakutō
Having merged with his Zanpakutō, Baishin is capable of transforming his arms into blades. He can then fire them at his opponents, allowing him to fight both at close combat and at a distance. Baishin seems to have the ability to call upon Hollows, but it may just be that they are attracted to him because of his spiritual energy.
 * Shikai: The name and release command of his Zanpakutō are unknown, due to the fact that when he is first introduced he has already merged with it.
 * Bankai: During the battle with Ichigo after absorbing a large amount of the formers energy, he is able to release his Bankai. In his Bankai form, Baishin's body becomes covered with hundreds of blades that act as both a weapon and armor. He has also shown the ability to shoot these blades to great effect. It is unknown whether this is his first time he used Bankai or if he just needed the power boost that Ichigo provided via his own power.

Quotes

 * "I am a Blade."