Shinsengumi

The Shinsengumi (新選組 or 新撰組, "Newly Selected Corps") are a special shinigami force formed after the defeat of Aizen. Realizing how vulnerable Soul Society would have become if Aizen had not been stopped by Kurosaki Ichigo, the Spirit King decided to set up a special unit along the lines of the Zero Squad which would strengthen Soul Society's defenses. But the true motive behind the creation of the Shinsengumi was for the Spirit King to have a special force present within Soul Society who would answer only to him and no one else. As such they are completely independent of Soul Society military hierarchy, answering only to the Spirit King.