Kaizō Konpaku (Seireitou)

Kaizō Konpaku (改造魂魄, Modified Souls), or simply known as Mod-Soul, are artificial souls designed to enhance regular Human physiology, making them capable of battling Hollows equally. The technology was created by Ōko Yushima, under the guise of Reigai (霊骸, Spirit Body), which are artificial spiritual bodies that allow modified souls to maintain a physical form in the Soul Society, among other dimensions.

Overview
Modified souls were created through Project Spearhead (尖兵計画, sempei keikaku; literally "Vanguard Plan"), in an attempt to even the gap between Shinigami and Hollows, as Hollows generally out number Shinigami. The Mod-Souls created varied in their abilities. For example, one Mod-Soul may be able to run many times faster than a regular Human while another may be many times stronger than normal. Kon, the Mod-Soul accidentally ending up in Ichigo Kurosaki's possession, has enhanced leg-strength. The Mod-Souls, just like regular artificial souls, are condensed into tiny, candy-like orbs. However, their originally intended purpose was to be placed into corpses to achieve their function as soldiers to combat Hollows. To fit with this purpose, they are also more independent than regular artificial souls.

The Mod-Souls were exterminated after the experiment was halted due to ethical reasons, since they were intended to be used to reanimate Human corpses to be used as weapons against Hollows. Yushima created the Mod-Souls and hid the technology within the Dangai when Project Spearhead was discontinued. He begins to take samples from highly ranked Shinigami as they travel through the Dangai, as well as reishi residue from major battles and events that had occured within the Soul Society, the Human World, and even Hueco Mundo. He uses these samples to combine them with his Mod-Soul technology to create enhanced clones of the Shinigami. He then places these clones into Reigai so that they can infiltrate the Gotei 13 in order to put his plans into motion. He places a bracelet on his Reigai to limit the spiritual power of the Mod-Souls, which can be taken off at anytime to unleash their full power.

The enhancements can come in several forms depending on the Shinigami's powers and the clone's appearance can also be modified if the physical profile of the original Shinigami is undesirable. Also, these Mod-Souls do not die if their Reigai is defeated, but only if they are destroyed in the Red Pill form that they reside in while not within the Reigai. These Mod-Souls are also unique in that their eyes have violet flashes in them while energized that can help distinguish the clone from the original. Once his research was complete, however, he had began to create unique Mod-Souls and stored them into specially-made Reigai.

Role of the Reigai
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. Yushima noted that, in the final stages of development, the Mod-Souls were but one part of the formula, as the Reigai used to house the soul would be the key in giving it form.

After being perfected by Yushima, a Reigai can be used to house unique Mod-Souls that possess special abilities as scripted by their creator. He had crafted these perfect Reigai bodies to encompass abilities and characteristics that are significant in species such as the Quincy and the Bounts. In the most general sense, these perfected Reigai have the capability to utilize reishi in a way that completely surpasses the aforementioned species.

Behind the Scenes
The author has chosen to completely disregard the filler Reigai Uprising arc, but will re-examine the Reigai concept for a new possible arc of similar features.