Tanin

Tanin (他人, Tanin lit, "Stranger") is the fourth Sankishin Zanpakuto forged by Tazuma. He developed specifically her for a highly spiritually aware human named Ryo Hiroshi without the need for Ryo to become a Shinigami, even going as far as melding Ryo's soul during the forging process, allowing Tanin to manifest herself in accordance to Ryo's own will to protect.

Appearance
Tanin's appearance resembles that of Yui Takasu, whom Ryo had loved and subsequent lost in a Hollow raid. According to Tanin herself, her appearance manifested due to Ryo's wish to gain power in order to protect the innocent, while Ryo also surmises that her appearance is the result of his soul yearning for Yui.

Zanpakuto
As of now, Tanin's Shikai and Bankai abilities have yet to be discovered. However, like Tazuma's other crafted Zanpakuto, Murakumo (spirit) and Totsuka (Spirit), Tanin is capable of physical Manifestation in the real world.