Bleach has an especially defined combat system for a shonen series. The Soul Reapers all carry Zanpakuto, or soul-cutting swords, which can transform into Shikai mode to gain new abilities, and some can transform again to manifest as Bankai. But that's not all: Soul Reapers can also fight their enemies with kido spells.
Only the Soul Reapers can use kido spells, though the Quincy have their own techniques and Hollows use their natural (and bizarre) abilities in combat. All Soul Reaper trainees are taught how to use kido, and the story's hero, Ichigo Kurosaki, has seen it in action more than once. But there's more to the kido system than just blasts of magical energy.
The Basics Of Bleach's Kido System
Kido is a soul's natural ability to use channel spirit energy outside their body into a spell. Even children can use basic kido spells, or at least manifest their spirit energy in a kido-like fashion. For example, in her street urchin days, Rukia could create impressive spheres of spirit energy, as could a young Renji Abarai. At the Soul Reaper academy, cadets are taught not only how to use the sword, but also how to properly summon and control their spirit energy. From there, a cadet may be taught the incantations for certain kido spells. Once a kido spell's incantation and name are spoken aloud, the spell will activate and take effect.
Kido spells are divided into two main categories: Hado, or destructive spells, and Bakudo, or defensive spells. Such spells are numbered 1-99, with a separate list for each of the two types. Higher-numbered spells are more effective, but also trickier to cast. Spells in the 90's are the most powerful of all -- only seasoned kido experts could ever hope to use. In Bleach's earlier episodes, Rukia could cast spells in single digits through the 30's, and near the end of the Soul Society arc, the traitorous Sosuke Aizen was casting spells in the 90s's.
Expert kido users can get creative with their incantations. Incantations are like short verses of poetry, and these arcane words allow the kido spell to take maximum effect. Low-level users need those incantations, but experts might skip them entirely and only say the spell's name, which (at the cost of some power) allows the spell to be cast much more quickly. If an enemy attack is incoming, the caster may not have the time to recite eight lines of poetry. But it is possible to combine incantations to cast kido spells one after the other, as Rukia did in the Hueco Mundo arc, taking the Espada Aaronierio Arurruerie by surprise.
Finally, while ordinary Soul Reapers can mix kido spells into their combat styles, the Soul Reapers who belong to the Kido Corps specialize in these spells, using them in place of Zanpakuto. In decades past, Tessai Tsukabishi and the future Visored Hachigen Ushoda both belonged to the Corps, and were on friendly terms with Kisuke Urahara, himself a master of kido.
Examples Of Bleach's Kido Spells
The Bakudo spells are defensive and binding in nature, designed to restrain Hollows or troublesome bystanders during combat. In fact, a Bakudo spell appeared in Bleach's first episode, Bakudo #1, Sai. Despite being so low in the ranks, this simple spell is useful for restraining a single person, usually by binding their wrists behind their back, as though bound by handcuffs. Rukia used this spell on Ichigo when he tried to interfere with her mission to slay Fishbone-D, but Ichigo broke free with his sheer willpower and spiritual energy, shocking Rukia.
Rukia and her big brother Byakuya Kuchiki used a few more restraint-oriented spells, such as Bakudo #4, Hainawa, to create a snake-like spell that restrained Aaroniero's sword arm to buy Rukia some time during combat. A more impressive Bakudo spell is #61, Riku Jokoro, which takes the form of six gold rods of light that restrain the target by their waist. Byakuya is rather fond of that spell, and Rukia also used it (with the incantation) to capture Aaroniero.
Once that Espada was locked in place, Rukia blew apart a wall to expose Aaroniero to sunlight and destroy his Kaien Shiba disguise. Later still, Byakuya used Bakudo #81, Danku, to create a tall, wide barrier that blocked the body-control effects of Zommari Rureaux's true form. Danku can block any kido spell level 80 or lower, and it can be created at a moment's notice. During a flashback, Sosuke Aizen also used Danku, to cover his retreat when Tessai tried to blast him away with a Hado spell.
A variety of Hado spells have been featured too, and many of them take the form of blue or red lightning and fire. One of Rukia's favorite Hado spells is #33, Sokatsui, which launches blue fire from the user's palm (Byakuya has been seen using it, too). Hado #31 is Shakkaho, or "red-flame cannon," and Renji Abarai used this spell to deal direct damage to Espada #8, Szayelaporro Grantz (though Renji was caught in the blast of his own spell, too). Meanwhile, despite having a low number at #4, the spell Byakurai can be deadly in the right hands, firing a thin, focused white beam of lightning. Byakuya used that spell to injure Ichigo's shoulder during their duel, leaving Ichigo unable to swing his sword.