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 and some hair of its head. 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.

Variations
Kisuke Urahara experimented with Gigai at length, and created at least two variations. The most notable one is an untraceable Gigai which drains the user's spiritual power instead of restoring it, effectively making them Human if used long enough. He also developed an inflatable Gigai for use as a decoy in combat.