Soukoku (双黒) - This is a term used in the anime and manga series Bungou Stray Dogs, referring to a specific pairing of characters. It translates roughly to "Double Black" or "Twin Black."

The ship cut through the leaden waves of the sea, leaving a foamy farewell trail behind. The young captain, Dazai Osamu, stood at the stern, his long cloak fluttering like a mourning flag. His eyes, the color of faded gold, reflected the endless water and the endless longing. The sea was his only love—fickle, dangerous, and never reciprocated. It was also a mirror in which he tried to discern meaning, a meaning that forever eluded him, like a fish in the deep.

Through the roar of the wind and the creaking of the mast, a different sound reached him—not the song of the storm, but a pure, piercing note, full of wild melancholy and equally wild hope. He walked to the side, peering into the foaming crests. And he saw it.

Amid the waves, a halo of wet red hair, lay a face of incredible beauty. Young, with sharp features and enormous blue eyes, swirling with a tsunami of emotion. Below, where the human's chest began, the powerful flesh of a mermaid shimmered like the scales of a magma-red tail. The creature was entangled in a net someone had cast, and the thin, strong threads dug into its skin and scales, staining the water pink.

Dazai ordered a halt. His crew, accustomed to their captain's eccentricities, lowered the boat without further ado. Pulling the merman onto the deck was a daunting task—he thrashed his tail frantically, baring his teeth and hissing like a sea demon.