Both mangas involve two princes of the underworld. Similarly, instead of being your typical, bloodthirsty demon, each have sympathy for humans and their fellow denizons, trying to build a world where both can live harmoniously.
Nurarihyon no Mago and Defense Devil both feature a young male protagonist who possesses an ability to transform, among other characteristics. Both mangas are lengthy, shounen, and are interesting due to their repective plot designs and character growth. Both stories boast a well designed world that is relevant to the main character and keeps you interested.
If it had to be said, even though both series have serious elements, defense devil felt more serious to me.
Power corrupts, and absolute power corrupts absolutely. Top-ranked student Yagami Light is disenchanted with the world around him. Through a series of events, he comes into possession of the ultimate power over life and death: a supernatural "death note" which can kill nearly anyone at his whim. As Light sets off on a crusade-turned-killing-spree, investigators from a police task force try to stop the mysterious deaths - including Light's own father, a senior policeman.
idk, there is just something about the main characters which they can relate. Both deal with a theme of justice. Defense Devil isn't as intricate as Death Note, but it does add a little bit more action/fantasy as well as a better FUCKING ENDING then Death Note does... in my opinion of course