Miko

The Miko is the title given to the strongest Exorcist.

Summary
Rokuro's enchanted guardian is Abe No Semei. There is plenty of proof for this and most people already know this. Kuzonoa calls Rokuro Semei. An impurity calls Rokuro Semei (zemei)(happens in chapter 3). In chapter 9 Mayura in impurity form says Semei while looking at Rokuro. The seal on his chest when his guardian is released is Semei's seal.

Benio's enchanted guardian is revealed by Kuzunoha to be Ashiya Doman, Abe no Seimei's archrival, with powers that match a Basara.

Rokuro's guardian represents yang and Benio's Yin. We can see this by the seals on their backs when their enchanted guardian is released. This leads me onto who the miko is.

Miko is and exorcist who has powers over both ying and yang. What Yuuto calls a true exorcist in chapter 14.