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."

Dazai walked down the hallway, staring at his phone. Chuuya came towards him with a stack of documents.
Chuuya always wore a cloak and hat (hence hiding his ears and tail from everyone), and he masked his omega scent with perfume. And everyone thought he was a beta.
Dazai crashed into the redhead. The omega fell, his hat flying off, and his cloak flying off. Dazai was taken aback when he saw the ears and tail and stood there, stunned.