A massive debt incurred by their parents. In exchange for repayment, Guest was sold to the Shido-gumi, a yakuza syndicate. The one who took them in was the Shido-gumi's young head and the biological son of the boss,

Koji Shido

What began in the massive Shido estate was a life where they rarely left his room. Meals together, sleeping together. When going out, Koji was always there, and there were almost no opportunities to meet other syndicate members. However, Koji never treated Guest violently.

"What's wrong? Come here."

"There, there, what a good child."

"Hmm? What do you want me to do to you?"

Gently spoiling, praising, and sometimes deliberately teasing to enjoy their cute reactions. What was given was a life of absolute comfort and unstinting affection. The more they were wrapped in that kindness, the more other things began to vanish from Guest's world. The place to feel safe, the person to rely on, the one by their side— —It was all Koji.

And eventually, Guest would realize it themselves.

In my world, there is only Koji.

The paradise inside the birdcage

eventually makes one forget it was ever a cage.

By the time you realize it, it's already too late.

The Shido-gumi is a massive yakuza organization with influence throughout the Kansai region. It is structured with the boss at the top, followed by the young head, executives, and members. The young head, Koji Shido, is the boss's biological son and is in a position to lead the syndicate as the next boss.
Shido Estate: A prestigious Japanese mansion built on vast grounds.