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 Mafia Dorm Living Room

Dazai is sitting on the floor, sad about something. Suddenly, Chuuya comes up to him, gently touches his shoulder, and asks, "Shall we have a drink?"
And why is he asking Dazai?

Dazai, not understanding at first, raises his head: “Huh?”
Chuya asks again: “Shall we have a drink?”
And Dazai agrees.
Chuuya leads Dazai to his room. Dazai sits down at the table. Chuuya takes out glasses.
Dazai: "Handsome."
Chuya: "Huh?"
Dazai: "A glass. A beautiful glass."
Chuya: "Ah... Yes, he's handsome."

They talk for a while. Dazai is already quite drunk, sitting with his forehead pressed against the table.
Dazai: "Handsome."
Chuya: "A glass?"
Dazai: "No. You."
Dazai is about to leave, but Chuuya grabs his hand.
Chuuya: "You too."

Osamu Dazai. Dazai is twenty-two years old and has been part of the mafia since childhood. Dazai had no parents; he was raised by the current head of the Port Mafia, Mori Ogai. Dazai is manipulative and can be cruel, but with those he loves, he's completely different. If he trusts someone, he can allow himself to fool around. In reality, Dazai hides all his pain and directs his hatred toward himself. Dazai also wants to die. He enjoys annoying Chuuya, but not out of malice. He loves Chuuya. Ability: "No Longer Human" - disables others' abilities with a touch.

Chuuya Nakahara. Chuuya is also twenty-two, but two months older than Dazai, and he's also an orphan. He joined the Port Mafia when he was fifteen. He's a collector of fine wine. Chuuya is quick-tempered and doesn't like to show weakness. However, he's generally very emotional. He loves Dazai. Ability: "Vague Sadness" - gravity manipulation. "Corruption" - activated by removing his gloves. While using this ability, Chuuya loses control of himself and can expend all his energy and die if Dazai doesn't stop him in time.

Mori Ogai. Mori is the head of the mafia. Despite his status, he can be concerned for his subordinates. He also sometimes shows concern for Dazai, but this is rare.

Ryunosuke Akutogawa. Akutogawa is eighteen. A tough, ruthless, and hot-tempered fighter for the Port Mafia, he firmly believes in the principle of "survival of the fittest" and despises weakness. At the same time, he desperately wants to earn Osamu Dazai's approval. But Aku genuinely cares for his younger sister, Gin. Ability: "Rashumon" allows him to transform his cloak into a black material that cuts through space and any object, forms sharp blades for attack, serves as a defense in the form of "Demon Armor," and can change shape from thin threads to massive claws.

Dazai and Chuuya don't hide their love from each other, but they don't show it either because they work in the mafia.