It was stated in databook.

Plus there are some section of people having rare blood.

I remember reading it in manga as well.

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It's explained very briefly in the manga, but genya happens to have a unique ability to gain strength from eating demon flesh (or things that come from.

Webhe's not a demon.

Webin chapter 136 genya says gyomei figured it out and even told genya to go to shinobu so she could examine him and make sure his body wasn't negatively affected.

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