私の お兄ちゃん が こんなに バカ わけ が ない!

Born to relatively normal oni parents Tsukiko (a general) and Natsune, (frequently found nude in the forest) Nana was raised by the two of them and his materal grandmother in the woods. Although his parents had intended to settle down and start a family, Nana proved to be a bit much for them. Tsukiko took her frustrations out in battle, while Natsune often disappeared into the woods (to be nude.)

Nanatsu proved from a young age to have powerful abilities, including metachrosis, (the ability to change color) epimorphic regeneration, (the ability to regrow, or grow new limbs) and very loud shouting. He also had an undeniable desire for human flesh. Altogether, he was a difficult kid, and hard to keep track of, let alone raise.

Tsukiko took to opening portals into the human world, luring humans over, and then feeding them to her family. When Tsukiko realized that Nanatsu had developed similar portal creation powers, she put a stop to things, battling her young son and ultimately forcing him to quit human meat cold turkey.

Overall, he was obnoxious, smart, and too powerful for his own good. Nana would slip away to the human world to steal from Hot Topic and antagonize managers at Denny's by the time he was a teenager. Mostly, he was lonely. There weren't many young oni in his community, and messing around in the human world was the only thing he could think of that was fun.

At one point, while in his own world, Nanatsu perished while out in the woods. He spent quite a while in the afterlife, where he made friends with a few other youths from his planet--Karsu, Suda, and later Mar. Eventually, the lot of them decided to return to the world of the living--being dead was very boring--and Nana harassed a passing priest into returning his soul to his broken corpse.

When he returned, he found that several years had passed, and his parents had news. They were expecting! Or, rather, had been expecting, for several years now.

Nanatsu was to have six sisters, or so an old prophecy had said. And, while he had been dead, two of them had hatched already. Both girls had been raised by other families, as was custom, but would soon be making the journey to the Nakai family's abode.

What Nanatsu didn't realize is what the prophecy meant. Each of the seven of them, him and his rapidly approaching siblings, were said to hold great power, enough to bring about the end of the world. And, eventually, they would.

In the meantime, at least he wasn't bored anymore.