One of the most unique elements of Bleach is Zanpakuto, as well as its two released states: Shikai and Bankai. However, in some cases, it is still unclear exactly what the difference between these two states is and what distinguishes Bankai from Shikai. Luckily, Tite Kubo has a great answer that succinctly explains the difference.
In Bleach, every Soul Reaper possesses a Zanpakuto, a special sword with a soul attached to a single person. Most Soul Reapers have the ability to use a power called Shikai, which allows them to use any special abilities the sword may have. Very few Soul Reapers have access to Bankai, a different type of release that is much more powerful than Shikai, generally speaking. But what makes a given release Bankai, rather than Shikai?
Tite Kubo's answer reflects not only the nature of Zanpakuto, but also of Soul Reapers themselves and the relationship between the two.
The power of Shikai and Bankai comes from different sources.
The two states of liberation are separated by the source of their power.
In a question posed on the Kubo's Klub Outside website, translated by Bleach fan forum user MShadow, a user asked why Hitsugaya was still able to use his Shikai after his Zanpakuto's soul was stolen by the Quincy's Bankai sealing medallion. Kubo simply replied, “Shikai is a state where the user's power is reflected onto the sword. Bankai is a state where the ego of the sword that was later born has developed and manifested.” This explanation actually fits with what we know about how Zanpakuto are formed.
Zanpakuto are born from an empty soul, called an Asauchi, which is then placed into a sword. Swords are given to Soul Reapers during training, and as the Soul Reaper spends time with their sword, it begins to evolve to match their innate skills and abilities. If releasing a Shikai is the user projecting their power onto the Zanpakuto, this would not only explain why Hitsugaya can use a Shikai without a soul, but also explain why most Soul Reapers unlock their Shikai long before their Bankai: projecting one's own power onto a relatively empty sword is much easier.
It is known that achieving Bankai requires a great deal of training – often decades, if not centuries, for most Soul Reapers. It is clear from Kubo's statement that part of the reason why Bankai is so difficult to achieve is because it requires a certain level of bond between the Soul Reaper and their Zanpakuto, a process that often takes a long time. As the Zanpakuto's spirit is gradually transformed, it becomes more and more aligned with the wielder's own power, until eventually the sword and wielder are both capable of using Bankai.
Much is known about the creation and function of the Zanpakuto, but this information clarifies a very important difference between the series' two most popular powers. While Kubo's explanation is excellent in its simplicity, it's a shame that it's never directly stated in the story. Bleach.
