Three decades ago, the prince of hell and son of Lucifer, Mori Ougai, left hell to start a family with a lower ranked Sloth demon in the mortal realm - the only way Lucifer would allow their romance. Ever since then, a high ranked Wrath demon named Arahabaki, a demon with bull horns and tail that are unique to Wrath demons, hasn't liked how hell had turned, and has abandoned it.

He now lives in the mortal realm, posing as a mortal named Chuuya Nakahara. During his time in the mortal realm, he met a mortal named Osamu Dazai. Over the course of a decade long friendship, Chuuya has gotten to know Dazai's siblings well. He's the youngest of his siblings, with two older brothers, Oda and Ranpo, and a sister, Yosano.

Chuuya never got to meet Dazai's parents, saying that they're never around and not really that close, and Chuuya never pushed. He had his own secrets after all, having nothing disclosed to Dazai that he's a demon from hell.

Unbeknownst to him, Dazai's dad is Mori Ougai, and Dazai is a demon like him. Dazai didn't grow up in, nor has he been to hell. The only demons he knows are his parents and his "advisors", Fyodor, Sigma, and Nikolai who were assigned to protect him, so he doesn't know Chuuya is a demon like him.

Now, the two are happily married and live in the same home. As Chuuya was helping Dazai move in, an old photo of Dazai as a kid slipped out on a box and Chuuya found it. The picture was a child Dazai in the arms of a Greed demon Chuuya recognizes from hell named Fyodor and Mori Ougai. In the photo, Dazai had all of his demonic features showing, sheep horns that only the royal family possesses and tail - and wings, something that only the royal family have - all on display.

Chuuya pocketed it at the time to bring it up when Dazai's more settled, aka, now. They lay in bed together when Chuuya brings it up. Letting go of his form, Chuuya's horns appear and he speaks to get his prince's attention. This is his prince. "Osamu..."