One of the most unique elements ofBleachis the Zanpakuto, as well as its two release states: the Shikai and the Bankai. However, in some cases, it hasn’t been clear just what exactly the difference is between these two states, and what separates a Bankai from a Shikai. Fortunately, Tite Kubo had a fantastic answer that succintly explains the difference.
InBleach, every Soul Reaper has a Zanpakuto, a special sword with a soul that is bonded to one person alone. Most Soul Reapers are capable of utilizing the power-up known as Shikai, which allows them to utilize any special abilities the sword might have. Far fewer Soul Reapers have access to Bankai, a different style of release which is far more powerful than Shikai, generally speaking. But what makes a certain release a Bankai, and not a Shikai?

Tite Kubo’s answer reflects not only the nature of Zanpakuto, but of Soul Reapers themselves, and the relationship between the two.
Shikai and Bankai Power Come From Different Sources
The Two Release States Are Separated By The Origin of Their Power
In a question asked on Kubo’s Klub Outside website, whichwas translated by user MShadow of theBleachfanverse forums, one user asked why Hitsugaya was still able to utilize his Shikai release after his Zanpakuto’s soul was stolen away by the Quincy’s Bankai-sealing medallions. Kubo answered simply,“Shikai is the state in which the wielder’s power is reflected onto the sword. Bankai is the state in which the ego of the sword that was subsequently born has been developed and manifested.“This explanation actually fits well with what’s known about how Zanpakuto are formed.
Zanpakuto are born from a blank soul, known as an Asauchi, which is then placed into a sword. The swords are given to Soul Reapers in training, and as the Soul Reaper spends time with their blade, it begins to evolve in response to their innate skills and abilities. If the Shikai release is the wielder projecting their power onto the Zanpakuto, it explains not only why Hitsugaya was able to use Shikai without the soul, but also explains why most Soul Reapers unlock their Shikai long before their Bankai: it’s much easier to project one’s own power onto the relatively blank sword.

Obtaining Bankai is known to involve a lot of training–often decades, if not centuries, for most Soul Reapers. It’s clear from Kubo’s statement that part of the reason that Bankai is so difficult to obtain is that it requires a certain level of bonding between a Soul Reaper and their Zanpakuto, a process which generally takes a very long time. Since the Zanpakuto’s soul is slowly modified, it becomes an increasingly better fit for its wielder’s own powers, until eventually the sword and wielder become capable of Bankai.
Much is known about the creation and function of Zanpakuto, but this information clarifies a very important distinction in two of the series' most common power-ups. While Kubo’s explanation is brilliant in its simplicity, it’s a bit of a shame that this is never spelled out directly in the story ofBleach.
Bleach
Bleach is a multimedia franchise created by Tite Kubo. The franchise started as a Japanese manga series. Ichigo Kurosaki is the main protagonist who is granted the powers of a being called the Soul Reaper. The success of the manga led to two anime TV shows, four feature-length animations, and several video games.