Gigai

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

Overview
Gigais are artificial bodies which 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 Gigais existed at least 101 years ago, as Kisuke Urahara was known to be experimenting with different variations of Gigai for several reasons.

A Shinigami might use a Gigai if 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 Hollow. and so choose to reside within a Gigai and begin to behave like Humans in order to blend in and avoid being hunted. They may use a Gigai 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 which 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 the appearence of its host, who can now be seen by normal Humans they happen to be around while residing 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 Gigais by a variety of methods, but the fastest appears to be using a Gikongan to replace the soul within, allowing it to freely leave the Gigai; in the case of a Shinigami, this is done so they may fight at their full strength while ordering the replacement soul to keep the Gigai out of harm's way. It seems leaving the Gigai without this method can be troublesome for a Shinigami.

Variations
Standard: These Gigais seem to be used for a single purpose of harboring the soul of a Shinigami while it carries out prolonged missions within the World of the Living. They are used to harbor a Shinigami who has lost their powers and are awaiting them to return so 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 which 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 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 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 is effective enough to fool 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 untraceable to keep the user's reiatsu hidden from other souls who 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 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, unlike others, she could freely exit from. Upon leaving, the Gigai would turn into autumn leaves and disappear until she needed it later.