Zanpakutō Spirit

Because a Zanpakutō begins as an Asauchi able to become anything, the spirit that emerges once a Shinigami imprints their soul onto it can take almost any form, up to and including the likeness of its own master or of another person entirely. No two share a common look, and a blade split into two swords carries two separate spirits. Their forms can also shift while a master holds Bankai or uses a Shin'eiyaku. The range is vast: Zabimaru appears as a white baboon with a snake for a tail, Hyōrinmaru as a huge serpentine ice dragon, Katen Kyōkotsu as a pair of women, and Ichigo's Zangetsu as two beings, one a color-inverted double of Ichigo and the other an old man wearing the face of Yhwach from a thousand years past.

Though shaped by their master's soul, these spirits hold thoughts, feelings, and wishes of their own that can diverge from the wielder's. Every one of them, whatever its shape, is intelligent, capable of speech, and aware of events beyond the inner world. They innately know the names and techniques they and their masters can use, and only the spirit can pass that knowledge on, though it will refuse a technique that would harm it or first test whether the wielder is worthy. Their temperaments color how a sword performs: Katen Kyōkotsu withholds her full power unless she is in the mood, Hōzukimaru is so lazy that Ikkaku struggles to rouse his Bankai, and Zabimaru handed Renji only a weakened Bankai until he proved himself and earned the true Sōō Zabimaru.

A spirit is born alongside its Shinigami and perishes with them, yet its power is its own, and drawing on it in addition to the wielder's strength makes the sword mightier still. Reaching that power depends on contact, most often through Jinzen, a sword meditation in which the Shinigami merges their mind with the spirit's inside the inner world. Neglecting that bond weakens both parties, as seen with Kenpachi, who never heard his spirit's voice, and with Hitsugaya after Hyōrinmaru's Bankai was stolen. Spirits fused with other powers can wield abilities such as Cero or Blut on their own. Comparable manifestations exist beyond Shinigami, including Orihime's Shun Shun Rikka and Coyote Starrk's living weapon Lilynette Gingerbuck.