Over Soul

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. is 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:

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 is requires great talent to wield effectively. Weapon-type are usually reserved for Seirei-type ghosts. Next is the oversoul merge which the shaman and ghost are one and the same. The only example of this was Choco and Mick. It is very difficult to pull off since both the ghost and shaman must be synced the entire time the oversoul is used. The advantage is that the ghost is fully used in the battle and a direct use of all abilities of both shaman and ghost. 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.
 * Ghost-type: Normal-class spirit ally
 * Seirei-type: Master-class spirit ally
 * God-type: Kami-class spirit ally

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.

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.

The sizes and shapes 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 big redesigns since their introductions, and have become more contemporary as the series progressed.