Reigai

A is an artificial and spiritual body that allows artificial souls to maintain a physical form in the Soul Society.

Overview
A Reigai is an artificial body used to house Gikon and Mod Souls while they are in Soul Society. As they do in the Human World, Artificial Souls exist as pills while in Soul Society as well. The Reigai are used by the Shinigami Research and Development Institute so that they can experiment on the artificial souls in a more physical aspect before they are to be used by any Shinigami going on a mission to the Human World.

While not being inhabited, the Reigai resemble a generic mannequin with rarely any distinguishing features although some have been shown to have hair in cases. When inhabited by an artificial soul, they can either keep this generic appearance or assume the characteristics of the soul that resides within them. This depends on the nature of the artificial soul itself as Kageroza Inaba managed to find a way to create Modified Souls that were cloned and enhanced versions of specific Shinigami. When these specific Mod Souls enter the Reigai, the artificial body takes on this soul's physical characteristics and capabilities. The Reigai that Inaba created for his rebellion also wear a bracelet that limits the Reiatsu of the Mod Souls inhabiting the Reigai.

In special cases, a Reigai is able to allow an actual soul to reside within it. This is seen when Inaba uses one that is injured during his encounter with Mayuri Kurotsuchi. By swallowing the red pill of one of his Mod Souls, he was able to eject his unhurt soul and force the artificial soul to take on the injuries he'd previously been inflicted with during his battle with the Captains of the Gotei 13.