Zen was once a yakuza enforcer who would kidnap the relatives of his family's debtors and hold them hostage to force them into paying. Eventually, he screwed up one of these kidnapping in an unspecified manner and ended up getting expelled from his clan.
After his dismissal, he was approached by the Reapers, who offered to pay him to kidnap yakuza and ransom them back to their families.
After Kiryu has helped the HLA convince several members of the Reapers to defect and become civilians, Zen kidnaps a couple of HLA members and holds them hostage in a building on Park Boulevard. Zen demands that the HLA should disband and stop interfering with the Reapers' vigilante activities or the hostages will be killed.
Kiryu tracks down Zen to his hideout and attacks him and his goons to free the hostages. Ultimately, Kiryu wins the battle and frees the hostage, before taking a battered Zen to the HLA headquarters to be questioned about the Reapers' leaders.
As a reward for rescuing the hostages, Kiryu is paid 120,000 yen.