What is a Zanpakutō?
Zanpakutō
A simple sword? A tool to be used and thrown away once broken? Did Ōetsu Nimaiya create the Asauchi for fun? A joke? The 'spirits' in them are a conjuration of the mind?
It is no mere scrap of steel.
A Zanpakutō is a life partner; a soul partner. The Zanpakutō is connected to its wielder through a bond within the soul. The Zanpakutō is created at the same time a soul manifests. It grows and develops along with its partner, even when the wielder is unaware. A Zanpakutō knows when its partner is in turmoil, for it understands. It understands and accepts; understanding and acceptance breeds trust. It soothes the wielder's worries, fears, and hesitance. Once the wielder knows, knows who is their Zanpakutō and what it is, it can be enough to bring them to tears of happiness and relief. For here is a being, it comprehends them, through all their faults and mistakes, it wants them to succeed, it will always be by their side, and encourage them to reach farther. It will accept future mistakes, help them through future problems, and be there, through hard and easy times. It is their Zanpakutō. The Zanpakutō cares for its partner's wellbeing of mind, body, and soul, even though with the wide variety of personalities it may not show. At least to outsiders.
The bond of Zanpakutō and wielder is one of trust, understanding, love, and so much more. It is a nigh unshakable, unbreakable bond. It's further than friendship, more than family, breaks through and above the limits of partnership. Soulmates. Not the kind with romantic aim, but where the intentions are not blinded by intimacy. They know what the other is thinking without a remark, can convey entire conversations without a word, and teamwork is perfect without a glance. Come what may, be it heaven, hell, or high waters, they will have each other's backs without the slightest hesitation or doubt. There is no other for them, to interpret them, to aid them, to endear them, to trust them, to embrace them. If one were to die, the other would follow. One cannot exist without the other, it's inconceivable.
Dangers are braved together, with their resolve strong.
Foes are fought together, with the wielder ruthlessly slashing, and the Zanpakutō keeping its edge sharp and unwavering.
Times of grief are mourned together, either silently comforting each other, no discussion required, or sobbing hopelessly together and clinging, while hearing easing words of solace.
Hardship is endured together, with the weight equally on both pairs of shoulders.
Joy is shared together, sending knowing smiles and laughter to the other.
Secrets are nonexistent together, nothing needing to be hidden.
This is a Zanpakutō, and the bond it holds with its wielder.
Unfortunately, these words do not, and cannot fully describe that bond. For mere words are unable to express it.
Frankly, I think the Zanpakutō in the series are just not touched on too much by Kubo. This is the explanation I was looking for, but never got in the series, or at least some deep meaningful explanation. Aka, I was not satisfied with those hints at what a Zanpakutō was. Filler does not count. Sure, Zanpakutō are pretty damn powerful if used right, and were explained, but the relationship part was just, meh. The series isn't over yet, but I'm not holding any hope for the explanation of the relationship of a Zanpakutō and the wielder.
My muse helped a lot with this, though she is a fickle one. And music, can't forget the music.