Puu defended Yusuke against reikai forces with ease without taking any damage. They are all A class in terms of spiritual power and one of them was able to take down Kurama in the past. So what I want to say is that the mazoku Yusuke we've seen from manga and anime wasn't even fighting with his everything yet because he wasn't fighting with his S class spirit beast at his side. We don't know how Puu can fight or if he can fight at all but his defense capabilities (something Yusuke lacks in his fighting style even as a demon) would be interesting to see combined with Yusuke's reiguns. Yusuke can ride on Puu's back and fire his shots while Puu deflects enemies with his wings. And if Puu can breathe fire or a spirit canon from his mouth, well. Tough luck on the bastards defying them. It would actually be interesting if YYH was allowed to have its last arc against King Enma go down like that. Yusuke as a demon taking down a corrupt god with the spirit beast that reikai used to test his heart. Makes the story come full circle. (Please refer to Dark Souls 3 Nameless King fight on Youtube for the visuals. I kept thinking about Yusuke and Puu while fighting that boss so now I need to bother an entire fandom with my blorbo thoughts.)