Kazuki Kasen is the older brother of Kenji and Asuka Kasen, both of whom would be the leaders of the Yakuza in Grand Theft Auto III.
In this game, Kazuki is the current head of the Liberty City Yakuza, who have only just begun to claim territory. He realizes that while all of the criminal organizations in the city fight over turf (mainly due to the actions of Massimo Torini), the Yakuza can move in and claim a decent amount of territory. Kazuki is well-dressed and well-groomed at all times. He is married to Toshiko Kasen by an arranged marriage, but neglects her, preferring the company of men. After his Yakuza suffer numerous setbacks due to Toni's actions, Kazuki decides to challenge Toni in a katana duel on the roof of Big Shot Casino. Toni wins, killing Kazuki.
In one mission, Toshiko claimed Kazuki is a samurai; this is a factual error as the military nobility had been abolished during the Meiji Restoration in 19th century Japan. However, it could also be assumed that Kazuki may have simply been a descendent of a samurai, or, more likely, followed the derivative of the Bushido code practiced by Yakuza, modified to fit their criminal lifestyle, and so is a "samurai" by way of practice and nickname, not in any official capacity within a literal government.
Kazuki Kasen is voiced by Keenan Shimizu.