The Yakuza is a criminal organization originating on Earth.
In the Books
In the books, the Yakuza employed Reileen Kawahara (TV) as an enforcer, where she would pour radioactive water down their throats.
In the TV series, the Yakuza seem to operate primarily on Harlan's World, with Tanaseda Hideki (a founder of Harlan's World) as their leader. After Takeshi Kovacs (Books) killed his father, Jaeger promised to find a good home for his sister Reileen, but instead, he sold her to the Yakuza.
At some point after this, another Yakuza boss acted in such a manner as to incur the wrath of the U.N.I.P., who dispatched C.T.A.C. to dispatch him. It was here that Tak found out about his sister and turned on C.T.A.C.
When Tak and Poe return to the hotel in 2414, they find that the same hotel is been used by Yakuza henchmen as a drugs lab.