In the final episode of the previous season, the cute fox drank a portion that allowed everyone to see him for some time. Kai could have done the same thing. But I agree, I think he is a powerful youkai
If it wasn't for the fact that him being a youkai explains how he thought that he was locked in the casket, I would have also considered the possibility of Natori being mistaken.
Since it is clear that he is a youkai, there are a lot of questions. What is his true motive? He is using Natsume or did he genuinely want to protect him. Also why was that blade youkai trying to kill him and what did the youkai mean by him wanting to kill the kid if he broke a promise.
There are many questions that arise due to that one revelation.