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Generally speaking, O.S. (Over Souls) are divided into three categories: Spirit-types, Weapon-types, and Armor-types. Spirit-types, in themselves, have three sub-categories: Ghost-types, Seirei-types, and God-types which can be molded into any form imaginable and possesses great power unique mainly unto them. There are also "Synthetic O.S.". These are not actually O.S., but artificially-created entities that can be materialized like an O.S (ie. the X-Laws' Archangels).
The Over Soul represents the ultimate form of furyoku manipulation, and depending on the ghost-type and skill required to engage an Over Soul, an O.S. can be categorized as one of three primary classifications: Spirit-types, Weapon-types, and Armor-types. The classification of an O.S. doesn't necessarily mean it will be more effective than a lower-class O.S., just more difficult to sustain, making the overall effectiveness of any O.S. largely based on individual talent. However, the types of ghost or spirit can alter the classification of an O.S.. The 3 main ghost types are as follows:
* Ghost-type: Normal-class shaman spirit ally
* Seirei-type: Master-class shaman spirit ally
* Kami-type: Kami-class shaman spirit ally
When creating an Over Soul, it's important for a shaman to make careful selection of what type of ghost they will use. This being because the ghost determines the properties and capabilities that an O.S. would possess. For example, Seirei-types, superior to general Ghost-types, possess greater supernatural abilities due to their evolved existence, making them perfect 'material' for a Weapon-type OS. Conversely, Kami-types are vastly more powerful, but the concentration and sheer strength needed to sustain them limit their use to only Kami-class shaman (ie. Shaman with a Furyoku value over 500,000FP). This is where comes into play. Reiyoku, essentially, is the measure of spiritual power & strength that an apparition (or ghost) possesses. This power is manifested in the form of magic and supernatural ability (ie. Kororo's ice magic). A ghost's reiyoku could mean the defining factor in a shaman, as a shaman must take careful consideration in when creating an OS with high reiyoku. This being because a ghost's reiyoku level determines how much of a shaman's furyoku it consumes in Over Soul mode. Even for Kami-class shaman, a ghost with a high reiyoku value could rapidly deplete their furyoku reserves in no time. So on the the part of the shaman, a Kami-class OS is restricted to a limited time in action as prolonged use would be disastrous for a shaman in a drawn-out battle (ie. Hao's Spirit of Fire: Kurobina). However, a shaman can filter the amount of furyoku consumed by a ghost by the type of OS they utilize.
Spirit-type O.S. are Over Souls which emphasizes the special powers of the spirit, using furyoku to give them form in which to act on the physical plane (ie. O.S. Bason, O.S. Kororo-Snowboard). Weapon-type O.S. manifests the ghost in their purest form usually in a symbolic offensive form (ie. O.S. Spirit of Sword, O.S. Kororo-Ikupasuy, O.S. Super Bushin), enabling the ghost to directly display their natural fighting capabilities as a weapon, but this type requires great talent to wield effectively. Weapon-type are usually reserved for Seirei-type ghosts. The final, and most efficient type is the Armor-type Over Soul. Armor-types represent the pinnacle of O.S. magic and are the most practical form of O.S. as they virtually impervious to attack due to their highly-density of furyoku, and are equally formidable in terms of raw power. In terms of sheer capability, Armor-types are perfect for shamanic combat.
More precisely, an Over Soul is a spiritual construction forged from the shaman's furyoku by materializing a ghost onto the physical plane via channeling media. An Over Soul 'actualizes' the knowledge and memories of the ghost and allows their memories to be manifested in a physical sense, as magical powers. They are composed entirely of pure furyoku and being so, there is no real matter to upon. This renders all form of conventional weaponry useless, as only another Over Soul can inflict damage upon another Over Soul. While any spirit can be molded into an Over Soul, the extent to which the spirit can be shaped depends on the spirit's ego--or physical attachment. Depending on their ego, the ghost can be molded by the shaman into any shape or form the shaman desire, enabling the direct display of their abilities to protect the shaman. Human ghosts usually make poor choices for an Over Soul due to their egos, but, over time can evolve into higher spirits--spirits whose souls with been refined for over hundreds of years and attained their purest form. The exact appearance of the Over Soul depends entirely on the shaman's imagination. But it's also important that the media that the shaman uses symbolizes the spirit's abilities.
Media is important for various reasons. They are easily identifiable with the spirit ally, thus making the transition easier to an Over Soul. But, in some advanced O.S. technique, a secondary medium is needed to stabilize and magnify a spirit's power that has exceeded the spiritual capacity of its original medium (ie. Yoh's "Double-medium" Spirit of Sword Over Soul). More precisely, a Double-Medium O.S. regulates erratic flows of furyoku that has externalized too fiercely. The shaman introduces a secondary medium as a stabilizing element to control the energy generated from the reaction of medium and ghost becoming an Over Soul, lest the shaman risks burning through their furyoku reserves too quickly. In similar fashion that the shaman's soul and that of his spirit ally are bound together through the O.S., combine the mediums into one large weapon. Thus, the Shaman can fully tap into his Furyoku potential, and deal much more damage over a bigger radius.
The strength and integrity of an Over Soul depends on the amount of furyoku it's charged with, and the shaman's willpower which generates the furyoku. It's important for the shaman to keep in mind that although the mass of an Over Soul remains constant, furyoku is consumed anytime it uses magic to create matter, like ice, which consumes furyoku in itself. Furyoku is only consumed when the "body" of the Over Soul is compromised. At this juncture, the Over Soul is charged with the proportionate amount of furyoku necessary to sustain an Over Soul, depending on the level of damage sustained. Furthermore, the full potential of an O.S. also rests upon the harmony between shaman and spirit ally, without which the O.S.'s efficiency could be compromised.
The size, proportion, and shape of certain character's O.S. often change since their introductions, some without any explanation. This could be attributed to the mental imagery a shaman uses to envision the form, size and ability of the Over Soul. Yoh's Spirit of Sword, for instance, has changed throughout the series since being introduce in volume 15 of the manga, with a defined blade and the inclusion of a (large) hilt. Others such as Ryu's Tokagero-Bokuto O.S. and Hao's Spirit of Fire are a couple of O.S. that have undergone numerous alterations since their introductions, with each change being more contemporary as the series progressed. On an interesting note, in the anime adaption of the series, Mikihisa introduces a technique called "Spirit Reduction" (a term coined by fans) which effectively reduces the size of a spiritual weapon without changing the amount of furyoku utilized thus bringing more power to their strikes. Canon only to the anime, this technique only applied to the also anime-canon "Giant Over Souls". Yoh and Ren were the only two, aside from Mikihisa, to acquire the skill; However, Hao demonstrated his mastery over the technique as well during his battle with Yoh.
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The technique was never named save by Mikihisa's technique, Spirit Reduction is a fan-term.
When they entered the Shaman fight, Yoh and his friends were all of a quite high power level : they were all able to use Giant Over Soul and in Yoh's case, Double Medium. However, they were still to the eyes of all no match against Hao Asakura, whose power was stronger than it ever was. It was these circumstances that brought Mikihisa Asakura, Yoh's father, to try to teach them how to be stronger. He showed them how his own very small spirits could easily disrupt their gigantic forms of Giant Over Soul, and made them meditate thereupon.
Only Yoh and Ren will benefit from this teaching, effectively reducing the size of their spiritual weapons without changing the amount of Furyoku externalized (for such is the secret of Spirit Reduction), thus bringing more power to their strikes.
However, Hao showed that he was also a master of this technique, as shown in the utter defeat of Yoh as he tried to stop Hao in a one on one fight.
They will abandon this technique completely during the final fight against Hao Asakura, Yoh found the larger sword more effective against Hao Asakura and Ren managed to merge his two Giant Over Soul forms.