Yukio is human but he was once possessed by Satan (when Yukio join/recruited by illuminati)
Idk it just in season one of the anime it looked like he had little powere that was his but okay ty tho hehe
It's been long since I've watched season 1 of the blue exorcist anime and if I remember correctly everything was Satan's doing to preven Yukio from harming himself. And we'll, at the end the studio had to change the ending a bit because it got ahead of the manga and couldn't keep with it so... Filler
The 2nd half of season one was all filler episodes.
Yukio is fully human, tho he has a natural resistancy towards demonic fire
Does the glasses twin ever awaken his demonic power?