>>723356449
The first game established him as a debt collector who would trash entire buildings and beat debtors to the brink of death. It's also shown that he trained Shinji to be pretty effective at collecting debts and possibly extorting money in other ways, as Shinji was easily able to afford luxury escorts. Plus he didn't mind his girlfriend working as a hostess, a glorified prostitute.
The second game had the bystanders remark on the fact that suddenly "someone" was drowning thugs in the Sotenbori river.
The third game has a substory in which Kiryu brings up his past in the debt collection business. Also when Mine visits him in the Purgatory, Kiryu is the only person in the room to shrug off Kanda's beheading and what Snake Flower had done to Hamazaki's family, implying not only he had seen some shit, he was completely used to it.
I can't remember which games featured them, but some other substories had Kiryu taking the side of loan sharks as long as they weren't charging exorbitant interest or constantly adding more fees.