Kodairei

The Kodairei (古代霊, Ancient Soul) is a title granted to only a small handful of individuals within the Yuengiri Order.

Overview
As explained by Kenji following his resurrection the Kodairei were those prodigal sons and daughters of the Yuengiri Order and the manner in which the Yuengiri passed on their most intimate secrets to their greatest scions -- knowledge which Sousetsu Shiba had removed from the Zhōng Yèshū before his death because he feared it falling into the wrong hands. The Kodairei was simply a title and not a race as Kenji originally believed, though he was correct in his belief that the Kodairei sat at the very tip of the Yuengiri Order.

Each of these individuals were tasked with building upon the Wan'nesu as a style for all and not simply mastery of it and of themselves as was the usual goal of its practitioners. This ensured that new skills and ideas were being brought forward. Only six Kodairei have ever existed in the past, with Kenji being the only current and each left behind a number of different techniques which Sousetsu and Aiko Shiba learned before removing them from the Zhōng Yèshū: their lasting legacy, now re-discovered by Kenji through his Sanzunokawa (さんずのかわ, lit. Sanzu River) technique and written in the Zhōng Yèshū where it belongs, once again.

In terms of physiology, the Kodairei are identical to. They use the same core abilities, possess a zanpakutō, and have increased lifespans. What sets a Kodairei apart from a standard Shinigami however is the nature of their powers. A Kodairei, for example, isn't a single soul; but two fused seamlessly into a single whole. To achieve the title Kenji needed to initiate a permanent spiritual fusion with the being sealed within him -- Kyoaku. Only then does a Kodairei come into existence. This practice allowed the oldest and most knowledgeable of the Yuengiri to fuse with the younger generation, thus preserving their wealth of knowledge and skill for future use. As such Kodairei also tend to life quite long lives, even by Shinigami standards. It isn't unheard of for them to live upwards to twice that of a normal Shinigami.