You mean what makes him irregular? 

We don't know as of now but there may be something special about him. Ma seems to be happy he is no longer one of the "gears" in the Evils Kingdom booklet which means that maybe Ma believed he would get in the way of her plans and she thinks he's gone for good but we have no idea really. 

The only other thing that makes him an irregular is the fact that he is the only person to appear in the Third Period from the Heavenly Yard but that wasn't really him as much as it was Sickle.

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