In the world of Bleach, Soul Reapers are tasked with purging evil souls known as Hollows from the living world, and use a variety of different means to achieve this. While the primary weapon used by Soul Reapers is a unique sword called a Zanpakuto, many also regularly employ spells known as Kido. Like activating a Zanpakuto, using Kido gradually drains a Soul Reaper's reiryoku, or spiritual power, so can only be utilized sparingly.
Types of Kido
Kido can be placed into roughly three categories. The first category is Hado, which consists of offensive spells. Hado spells are given a number from 1 to 99 (with higher numbers being more advanced spells), although the vast majority are unrevealed. Hado up to the mid-30s are fairly common throughout Bleach, especially because Rukia Kuchiki, a main character, employs Hado frequently in fights. Her preferred Hado, Number 33: Sokatsui, emits a bright blue flame which can eradicate lesser enemies. The most powerful Hado abilities are capable of taking down a Captain-level Soul Reaper in a single blow, demonstrated in brutal fashion when Aizen defeated Squad 7 Captain Sajin Komamura using Hado Number 90: Kurohitsugi, which trapped and severely injured the latter inside a black box of energy.
The second category, Bakudo, follows most of the same rules and conventions as Hado, but instead comprises defensive and support spells. Lower-level Bakudo like Number 1: Sai, and Number 4: Hainawa, are generally used to restrain enemies for a short time, while the strongest Bakudo are either powerful barriers or sealing techniques. Strong Bakudo, like Number 99: Kin, have been shown to subdue a Hollowfied Captain-level Soul Reaper like Kensei Muguruma with some difficulty.
The final category, Kaido, is separate from other forms of Kido in that the spells are not named nor given numbers, and consist of healing spells. These are primarily utilized by the members of Squad 4, which acts as the Soul Reapers' medical corps. Kaido is shown to heal moderate to serious injury, but is inferior to Orihime Inoue's Soten Kisshun, which can reverse events that have occurred, up to and including the loss of limbs.
Who is the Strongest Kido User?
Soul Reapers' individual aptitude for Kido varies widely. At the lowest, level, some soul reapers choose not to use Kido at all, including Ichigo Kurosaki and most of Kenpachi Zaraki's Squad 11, except for 5th seat Yumichika Ayasegawa. On the other side, a few Soul Reapers, like Nanao Ise, rely on Kido almost exclusively in battle. Some powerful Soul Reapers like Renji Abarai (a former member of Squad 11 himself), admit that their mastery of kido is lacking, and can only use lower-level spells with incantations. Experienced Kido users like Rukia and Momo Hinamori can cast mid-to-high level spells without incantations, in exchange for them coming out at a fraction of their power. A few current or former Soul Reapers have mastered Kido to such an extent that they can use nearly all the spells without incantation and with relative ease.
It has emerged as a trend, however, that with the exceptions of Byakuya Kuchiki and Retsu Unohana, the strongest known Kido users in Bleach are former Soul Reapers who have since left or been exiled from the Gotei 13. Approaching the top tier of Kido users are Sosuke Aizen, Kisuke Urahara (who can use Hado 99: Goryo Tenmetsu without an incantation), and the former captain and lieutenants of the Kido Corps, Tessai Tsukabishi and Hachigen Ushoda, both of whom have used high-level Bakudo without incantations.
Kido is a fascinating and often overlooked weapon in the Soul Reapers' arsenal, due to the fact that Ichigo and many main characters, as well as most of the villains, never utilize it. Additionally, the fact that only a fraction of Kido spells have been revealed gives Tite Kubo plenty of opportunities to expand on it in future Bleach media, which would be a great addition to this already lore-heavy universe.