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.
Kagome was just a normal girl, until one day she fell into a well that brought her to feudal-era Japan. After encountering hordes of demons and discovering that she's the reincarnation of a priestess, Kagome must now journey to protect a sacred jewel from evil with the help of a half-demon named Inuyasha. Along the way they make many enemies and friends, and travel between the modern and feudal eras.
In both manga the story is focused on the main characters which are part demon and part human. The main characters try to cope with not only their families but the people that want to kill them for various reasons (Inuyasha because of being a half breed and protecting the shikon no tama and Rikuo for being the next in line to be the nurarihyon). Also they are further troubled because of their impressive lineage, coming from the union of the strongest demon and a human takes its toll on these two protagonists by occasionally losing their sanity due to the demon blood within them. Furthermore these two characters resolve to prove themselves to the world and over time learns to accept their lineage.
Five hundred years ago there lived a feudal lord with a vast amount of spiritual power. To keep it from falling into the hands of demonic Ayakashi, the mage Tokimori Hazama and his two students were called to defend the land. However, they fell ill, and the feudal lord was killed and buried on the land now called Karasumori. Years later the successors of the Hazama students, Yoshimori and Tokine, have inherited the powers of the Hazama clan. They are Kekkaishi, users of the Kekkai, a secret and powerful technique that creates a strong barrier to capture and eliminate Ayakashi. Protectors of the power buried beneath Karasumori, Yoshimori and Tokine spend their days in the school built on the land, and their nights defending it from attackers who wish to use the power for their own purposes. With Karasumori's power growing more unpredictable everyday, will Yoshimori become strong enough to protect the ones he loves?
Both shows are shounen battle mangas, dealing mainly with monsters from japaneese folklore. Whether the enemy is called yokai or ayakashi it makes small difference, since they are spirits/monsters either way.
In both cases the main character is a power user coming from a very distinguished lineage of power users. Whether it's the grandson of the strongest youkai (Nurarihyon no Mago) or heir of the strongest barrier technique users family (Kekkaishi) both main characters are very reluctant to continue "family traditions". While circumstances pull them into the fights with monsters, they still try to lead a normal life, and hope for a normal future.
Both stories feature battles to protect their loved ones and exterminating the Evil. There are various supernatural elements in both series from Demons,Yokai's,Ninetailed Fox,Holy Priests in Nurarihyon and Dragons,ayakashis,immortal organisms and power user in kekkaishi
Middle school student Tsuna has always gone through life in last place; but when he learns that he is getting a tutor to help him raise his game, he had no idea that his benefactor would turn out to be... an infant and a mafia hitman?! As if things can't get much stranger, Tsuna then discovers that the powerful Vongola family has chosen him to be its next successor - a position that has control over all mafia families. Joined by friends Tsuna unwillingly starts out on his path to becoming a mafia boss - if he can survive the training!
Both Reborn! and Nuraihyon no Mago have a similar feel between them. They both are about young and weak boys becoming the next head of a top mafia family (Reborn of the human world, and Nuaihyon of the demon world). They both have personality changes, respected older "brothers" to guide them, and a lot of action. But putting all of that aside, they just remind me of each other, between the plots and rivals, and all the lives at stake. If you like one, you should give the other a try.