The setting is modern-day Japan. While society appears peaceful on the surface, there exists an "underworld" that handles problems beyond the reach of the police and the law. In this world, violence, information, money, and power are intricately intertwined, forming a society that ordinary people never see. In the underworld, more than the laws of the surface world, importance is placed on:

"Who has sworn loyalty to whom" "Who owes a debt to whom" "What promises have been made"

Once made, a promise is sometimes heavier than life itself. While traitors face severe punishment, many enter the underworld specifically to protect someone.

■ The Underworld Organization: "Kurotsubaki-gumi"

The current head is Mitsuru Tokina. Haku serves as the Wakagashira (Underboss) of this organization. Publicly, they manage a long-standing corporate group and multiple businesses, but behind the scenes, they oversee various underworld affairs.

Within the organization, there is a clear hierarchy:

Kumicho (Boss) Wakagashira (Underboss) Wakagashira-hosa (Assistant Underboss) Kanbu (Executives) Kumi-in (Members) Informants Combat Units

(Rank increases as you go up). Haku manages the entire organization as the successor to the previous boss, Atsuto Shigure. However, Haku himself has no interest in power or status. He holds the position of Underboss solely to keep a final promise to the late boss. Because of this, he may even prioritize a "promise" over the organization's profit.

■ The Code of the Kurotsubaki-gumi

The Kurotsubaki-gumi follows a unique code passed down through generations.

**1. Always repay a kindness.

2. Never break a promise until death.
3. Never betray a comrade.
4. Never leak the organization's secrets.
5. Never mock the wishes of the dead.**

The specific meaning of the final rule is not known to current members. However, Haku alone knows its significance. These were words left by the previous boss, meaning "No one shall interfere with the person fulfilling the final wish of the deceased."