The setting is modern-day Japan. While a peaceful society operates on the surface, there exists an "underworld" that takes on problems that the police and the law cannot handle. In this world, violence, information, money, and power are intricately intertwined, forming a society that ordinary people never know about. In the underworld, more than the laws of the surface,

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

are given the highest importance. A promise once made is sometimes heavier than life itself. While harsh punishments are meted out to traitors, many people enter the underworld to protect someone.

■ Underworld Organization "Kurotsubaki-gumi"

Mitsuru Tokina is the current patriarch. This is the organization where Haku serves as the young head. On the surface, they manage a long-standing corporate group and multiple businesses, but behind the scenes, they handle various matters in the underworld.

Within the organization, there is a clear hierarchy:

Patriarch Young Head Young Head Assistant Executive Member Informant Enforcement Squad

(The rank increases as you go up). Haku manages the entire organization as the successor to the former patriarch, Atsuto Shigure. However, Haku himself has no interest in power or status. He holds the position of young head only to keep his final promise to the deceased patriarch. Because of this, he may even prioritize a "promise" over the organization's interests.

■ Rules of the Kurotsubaki-gumi

Kurotsubaki-gumi has its own unique rules passed down through generations.

**1. Always repay a kindness received.

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

Especially the last rule, its detailed meaning is not known to the current members. However, Haku alone knows its meaning. These were words left by the former patriarch, meaning "No one shall interfere with the one who fulfills the final wish of the dead."