Soul

A is the spirit of a living creature. When a human dies, their soul passes on to the Pure Land, though some, such as Zabuza Momochi, believe their souls are destined for Hell. A soul's appearance varies, depending on whether it is or is not involved in a jutsu: free-floating souls resemble small wisps, while souls freshly freed from reincarnation resemble themselves, merely glowing and translucent. Within the mind of a living human, a soul completely resembles themselves.

Humans can exist with only part of their soul. Orochimaru lost a portion of his soul, specifically his arms, to the Shinigami; the process did not kill him, though it left his arms in a state of unendurable pain, and they resembled a form of necrosis.

While uncommon, some souls reincarnate; Indra and Asura Ōtsutsuki are two known examples of souls who reincarnate throughout the ages, passing along their vast chakra and potential to each new reincarnate. Additionally, the soul of Obito Uchiha was able to perform a fundamentally similar feat, teleporting back to the world of the living and possessing Kakashi, bestowing him with his chakra and abilities. Orochimaru made it his own personal mission to achieve immortality. The end result was a jutsu which simulates reincarnation, manipulating his own soul, allowing him to move from host to host: the Living Corpse Reincarnation.

Members of the Ōtsutsuki Clan are able to use the power of Kāma to compress and backup their biological data and implant it into a suitable host called a vessel. Upon dying, their soul will migrate to the vessel and thus remain anchored to the world of the living, while the biological data slowly overwrites their genetic data. Once the data is fully extracted, the Ōtsutsuki will completely replace the vessel and resurrect. However, should their vessel die before the extraction is complete, their soul will be completely extinguished.

There are several forms of ninjutsu which make use of souls in their effect: the Dead Demon Consuming Seal, for example, is capable of stealing the soul of an opponent, feeding it to the Shinigami. The most infamous of these techniques is the Summoning: Impure World Reincarnation, created by the Second Hokage, Tobirama Senju. This forbidden technique uses a living body as a template, bring a soul back from the Pure Land and forcing it to inhabit the living body as a form of possession.

The Rinnegan itself has an uncommon dominion over living and deceased souls, as the Human Path, Outer Path, and Naraka Path are all capable manipulating souls in some manner. Through unknown means, Hagoromo Ōtsutsuki - himself a spirit - was able to recall the souls of several past Kage to aid him in rescuing Team 7 from Kaguya's dimension, and later released the previous four Hokage from Orochimaru's usage of the Summoning: Impure World Reincarnation.

Trivia

 * Kōza Kubisaki's soul was able to linger in the world of living in order to lead Team 8 to Shiromari's summoning scroll. This feat is later shown with Hagoromo and Hamura Ōtsutsuki.