The Kenrei (剣霊; lit. sword spirit) was a group of rogue shinigami who were the direct result of a failed experimental project performed by the Twelfth Division, under the supervision of the Kidō Corps, 400 years before the main Bleach continuity. This project forcibly sealed shinigami zanpakutō spirits within its respective owner, essentially transforming the wielder into a weapon. The experiment was able to produce operative subjects, though the mortality rate was considered too great to be used on a larger scale and the subjects were ordered to be executed by the Central 46. The survivors of the terminated project fled to the human world with the help of Kenji Fujibayashi after Baishin, who was transformed into one independently of the experiments, went on a killing rampage that left over fifty officers dead.
The kenrei gradually came to be referred as a species separate of the shinigami, despite strong connections between the two. They took refuge in the human world, and generally were pessimistic of the Gotei 13 and Central 46 due to the execution order.
The experiment that created the Kenrei was meant to create a group of super soldiers by forming the closest possible bond between a shinigami and his or her zanpakutō, based on the unique attribute of Kenichi, the zanpakutō of Kenji Fujibayashi. A three-part forbidden Kidō provided by the Kidō Corps was used in order to make this possible.
The experiment failed, since only ten of the twenty selected shinigami, chosen from the Gotei 13, Kidō Corps and Onmitsukidō, had their zanpakutō successfully sealed within them, fusing to the shinigami's spiritual essence. The other ten participants were killed in the procedure when their souls were eroded away. The results were delivered to the captains of the Gotei 13, as well as to the Central 46 for consideration.
The shinigami Baishin caught wind of the Kenrei Experiments and was successful in illegally merging with his zanpakutō in a more literal form. He was lucky to survive the procedure, later going on a killing rampage which left over fifty officers, including lieutenants, dead. Even so, Gotei 13 captains were able to capture and imprison him, though two were killed in the operation.
The dangerous nature of Baishin prompted the termination of the other kenrei, out of fear that they would also retaliate against the shinigami. Kenji found out about the order and broke the test subjects out of their cells and fled to the human world with them by himself.
Powers and abilities
Main article: Wakai
Their abilities vary with individual, though the Kenrei share the common ability of combining their own skills with the powers of their zanpakutō. In doing so, a Kenrei essentially transforms him or herself into a weapon. They maintain a spiritual blade that could be used to fight shinigami or arrancar, however it disappears in most cases where the kenrei activates shikai. The Kenrei's release of their inner power is known as Wakai (和解; lit. harmonic release).