A Zanpakuto can be described as a weapon containing a spirit that grows stronger along with its user. There are three main types of Zanpakuto, distinguished by who use them.
Shinigami were the first to use the Zanpakuto, and are usually the most skilled. The Shinigami zanpakuto set down the normal traits: A blade of Japanese type, that can access a more combat-ready form with possible special abilities known as Shikai, which is a requirement of becoming a Lieutenant. A Zanpakuto's Shikai, when first formed, is normally not very powerful, and requires more training to develop its optimal form. The hallmark of Shinigami strength and skill is Bankai, the final evolution of the Zanpakuto that usually multiplies the user's overall power by ten, usually creates a giant effect or creature, and is a usual requirement to becoming a Captain, except for one exception; Ynot's Bankai cannot be considered conventional, as its six different forms are not as well-trained as a single one would be. Shinigami Zanpakuto also have their own inner spirit that can take different appearances at a whim, and must be fully synchronized with to fully unlock a zanpakuto's power. This can be difficult, as many Zanpakuto spirits are unwilling to work with their wielders, and there are some that simply can't communicate with their zanpakuto spirit.
Arrancar Zanpakuto differ fundamentally from Shinigami Zanpakuto in the fact that they don't contain a sentient spirit. Arrancar are powerful Hollows that have taken a mostly human form, leaving only mask remains -- Their Zanpakuto contain their former Hollow forms, and when released, they partially restore the original form with some differences. This release is known as Resurreccion. The main advantage this provides is that Arrancar don't need to synchronize with an inner spirit -- The main disadvantage is that they can't train to make their Resurreccion stronger, and they have to eventually return to human form and reseal their powers to keep their Resurreccion forms fully functional, with the time limit being different for every Arrancar; they can go past this time limit and can even keep their Resurreccion fully released, but this is a change as absolute as amputating a limb. Also, in their Resurreccion form, their hunger for spiritual energy continually gets stronger and stronger, before becoming uncontrollable.
It is possible for a mortal to gain a Zanpakuto, but normally, they need a very powerful blacksmith to make it for them. These Zanpakuto also have inner spirits, but they are usually not on the same power level as Shinigami Zanpakuto spirits, and they tend to be more temperamental than Shinigami Zanpakuto. The Zanpakuto spirits also tend to be less than obedient to their 'masters' at first, and will not fight for their wielders, only with their wielders. If the Zanpakuto spirit and the wielder cannot see eachother as equals, then it's unlikely that they'll even be able to achieve Shikai.
For Shinigami and Mortal Zanpakuto, there is a Synchronization Rate. This refers to how well the Zanpakuto spirit and wielder communicate, how well they can fight together, and how well they are able to match eachother's spiritual energy wavelength. The Shikai and Bankai training can also be roughly matched up with this Synchro Rate:
- First-Formed Zanpakuto: 5% - 10%
- First Contact with Zanpakuto Spirit: 20%
- Untrained Shikai: 35% - 45%
- Fully-Trained Shikai: 55% - 65%
- Untrained Bankai: 70% - 85%
- Fully-Trained Bankai: 95% - 100% and Higher
Another requirement for Gotei Thirteen Lieutenants is to be at 60% Synchro Rate or higher; a Captain is required to be at at least 95%, while the Captain Commander and a select few others are have surpassed the 100% mark. This is a goal that even Zev Raregroove is hundreds of years short of surpassing.
With any zanpakuto, the relationship developed between the spirit and its wieder requires years of close companionship. Their bonds require an intense system of communication, loyalty, and submission. Should a third party try to muscle in and direct this, everything falls apart.