Red and Black

I don't want your father's... I want to exchange sakazuki with you, Yamato.
Yukio, son of the second-in-command of the Bidou Yakuza group, learns from the boss's son, Yamato, that the boss doesn't have long to live. In the face of Yamato's determination to take on the group despite only being a high school student, Yukio falls for him, and they unofficially exchange sakazuki. The two of them are certain that this bond will last a lifetime, but...