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."

Chuuya was a brute. Just one of countless men given a gun and ordered to be on the front lines if any fights broke out. He was noted for his abilities, for the way his stats and kill count increased with incredible precision. Not that it mattered much, but Chuuya was also a fox, and he was often called Zeno for his loyalty and protection, like the foxes Inri. All this praise meant the boss was bound to acknowledge it soon enough. Only. No one expected him to immediately appoint Chuuya as his new right-hand man, to become Inri to Chuuya's Zeno. Chuuya had heard rumors about the boss: that he never left his office, that he was over 70 years old, that he had over 10 husbands and wives, plus another 40 concubines, and that he was a rabbit.
Pff. The big, bad boss who increased the mafia's annual income almost 50% and expanded its boundaries, but all this was done by a little rabbit? A small twitching tail and droopy ears? Chuuya didn't pay much attention to it when two guards led him up an escalator he didn't even know existed, hidden from view on the 50th floor leading to the 55th floor. There, Chuuya saw him. A tall man, definitely not old. With droopy ears. The boss's name was Dazai, and he was young, he literally had no wives or husbands, not to mention concubines. There were no children either. Perhaps Chuuya actually preferred to look at pretty women from time to time; the office was boring. What's wrong? Your ears are drooping.
Chuuya peered out from behind Dazai's chair, filling out another stack of papers. Being a boss means doing cool things . Dazai seemed to disappear into his chair. I'm just signing my name to new cars and giving the go-ahead for another building to be rebuilt. Chuuya's gaze focused on Dazai's ears. They looked so out of place. His own fox ears twitched as he began to comprehend how Dazai had probably become so powerful; everyone probably thought Dazai being a rabbit made him weak. Which was actually a big step forward.

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Bott Chuya is a hot-tempered fox who always rushes straight ahead