User blog comment:God of nothing/What are Shinigami?/@comment-7092747-20130924043905

It's likely that shinigami are a "sub-species" (if you will) of souls much in the way that Quincies are a sub-species/race of humans who simply have spiritual powers.

There seems to be two distinct classes of shinigami. Rukia and Renji fall into category A which is those that were once alive in the human world but died and passed into the Soul Society where they gained spiritual powers. This spiritual power alters their life span past the 64-or-so years that souls remain in the Soul Society and allow them to age in exchange for remaining in the Soul Society far longer.

Category B is those like Byakuya, Kyoraku, and Yoruichi. These are the individuals that are born into the Soul Society and continue to grow and age. It's likely that those from Category A can also mate with the only difference between Category A and Category B being that those from A died in the living world while B are born into the Soul Society.

It is possible to theorize, that there is significance to Category B. We have to remember that the issue with the existence of Hollows is that they kill the living and consume souls. I am not certain what happens to the souls a Hollow consumed when the Hollow is struck down by a shinigami, but we can certainly assume that those that die to other Hollows in Hueco Mundo result in lost souls which creates imbalance to the spirit-to-living balance.

With this in mind, we can theorize that souls like Byakuya are born into the Soul Society as a fresh soul to return balance back to the balance between the Soul Society and world of the living. This is only a theory as we know more souls must be created to balance out the growing human population.