Shinkūmyō (空の存在, "Empty Existance") are Souls that have been subjected to a strange phenomena that caused them to start transforming into a Blank but have resisted the loss of memory and individuality, becoming entirely new beings in the process. It is a concept that has eluded even the most brilliant of minds in the Research and Development Institute of the Gotei 13.
Blanks are, by nature, beings that follow some singular mindless sense of desire. Because they normally lack individuality and sense-of-self, they merely exist as masses of energy conjoined by some predetermined direction. The process at which this occurs normally happens when souls that have undergone the Konsō ritual by a Shinigami fall off the "path of reincarnation" and end up losing their memories as they drift away as white puffs of spiritual energy tied together by whatever emotions they were driven by. However, there exists a very strange phenomena when a soul undergoes this situation of losing their memories but somehow force themselves to retain their individuality. In doing so, even the memories that were temporarily lost return to the soul as they complete the transformation and turn into Shinkūmyō.
While quite an unusual circumstance, Shinkūmyō appear to develop almost exact physiology to that of Shinigami, in that someone that transforms into a Shinkūmyō may be capable of obtaining a Zanpakutō. In addition, should the soul that is subjected to the transformation happens to be a two-dimensional being, including that of Shinigami, Sōzōshin, Arrancar, and otherwise, normally keep the form and appearance they had prior to their failed transformation. However, in the case of a normal one-dimensional soul, they attain a long white cloak with ragged edges that cover their body similar to that of an actual Blank.
It has been shown that those that undergo this process often experience an alteration within their very soul. Almost as if their very soul is reconstructed, the Shinkūmyō experiences a change within them that turns them into the being that they could most identify and most closely relate with. In Seireitou Kawahiru's case, his soul became more like that of a kitsune, resulting in his unique transformations. However, not always does this change involve an animal, and in some cases, the reconstruction of their soul, at least in a Shinigami's case, can lead to changes that occur within the Zanpakutō they possess, ranging from miniscule changes to the birth of an entirely new Zanpakutō spirit altogether.