Gigai

A is an artificial body that allows Shinigami to remain in the Human World and interact with Humans.

Overview
Gigais are artificial bodies that are used for a variety of reasons by Shinigami within the World of the Living. It is currently unknown when exactly the first Gigai was formed and by whom. However, it is known that Gigais existed at least 101 years ago as Kisuke Urahara was known to be experimenting with different variations of Gigai for several sets of reasons.

One reason that a Shinigami might use a Gigai is in the event that they lose their powers as only with their powers can they return to Soul Society. However, while remaining the world of the living, they are targets for Hollows and so choose to reside within a Gigai and begin to behave like Humans in order to blend in and avoid being hunted. Another reason is if it is required for their mission to interact with Humans or to reside among them in order to search for threats and to blend in while doing so.

While being created, Gigai can be designed with specific attributes that make their jobs easier depending on their purpose. During this stage, they have the generic appearance of a muscled Human with no unique characteristics or features. When the Gigai is hosted by a Shinigami, it assumes its hosts appearance and they can now be seen by normal Humans the Shinigami happen to be around while they reside in the Gigai. The Gigai does not take on the clothes its Shinigami host is wearing and thus, they must acquire modern clothes in order to blend in with the Humans.

Souls can leave Gigai’s by a variety of methods but the fastest appears to be by using a Gikongan in order to replace the soul within, allowing it to freely leave the Gigai; in the case of a Shinigami, this is done so that they may fight at their full strength while ordering the replacement soul to keep the Gigai out of harm’s way. It seems that leaving the Gigai without this method can be troublesome for the Shinigami.

Variations
Standard: These Gigai seem to be used for a single purpose of harbouring the soul of a Shinigami while it carries out prolonged missions within the World of the Living. They are also used to harbour a Shinigami who has lost its powers and are awaiting them to return so that they can return to the Soul Society. They have been used by multiple Shinigami during the Bount Incident and between the numerous invasions of Karakura Town by Arrancar.

Unfinished Prototype: Before his exile, Urahara was shown to be experimenting and attempting to create a special Gigai. At the time, souls within the Rukongai were simply evaporating under mysterious circumstances and Urahara aimed to create a Gigai that would help souls remain in Human form instead of disappearing. The incidents were a result of experiments being done by Sōsuke Aizen and his colleagues. Before Urahara could make use of this Gigai in the way he intended, he was exiled and was forced to use this technology to help more victims of these experiments.

Reiatsu Concealing Gigai: This Gigai was developed 101 years ago by Urahara so that he, Tessai and the Visored could escape from Soul Society and live, undetected, within the Human World. During this time, Urahara aimed to find a way in order to reverse their Hollowfication as a result of Aizen's experiment, which saw them condemned to death.

Inflatable Gigai: Urahara developed this Gigai to use as a decoy in battle. It is seen being used against Yammy Llargo during the third Arrancar invasion of Karakura Town. It also appears to be effective enough to fool even Aizen when the two faced off during the Battle of Fake Karakura Town.

Rukia's First Gigai: This Gigai appears to be unique in that instead of allowing the soul to recover, it actually gradually lowers the host’s Reiatsu until they become a normal Human with no spiritual powers whatsoever. It was also untraceable to keep the wearer’s Reiatsu hidden from other souls that may be looking for the host. This Gigai was given to Rukia by Urahara in order to permanently seal the Hōgyoku within it, thus keeping its immense power, out of Aizen’s grasp. After Rukia's capture, Aizen made sure that the Gigai was destroyed.

Appearances in Other Media
During the course of the first film, Bleach: Memories of Nobody the main heroine, Senna used a unique Gigai, which she could freely exit from unlike others. Upon leaving, the Gigai would then turn into autumn leaves and disappear until she needed it later.