The Yakuza are the organized crime, the underworld of Tokyo. They are seperated into 3 main families of roughly equal power: the Yamaguchi-gumi, the Sumiyoshi-kai, and the Inagawa-kai. Descendants of Japan’s Burakumin (impure, second class citizens), the Yauza are steeped in traditions of death and corruption.
Inagawa-kai: The Inagawa have the least amount of actual power in Tokyo, in no small part due to the expansion of their operations outside the city. They have the most influence of the three families outside of Japan.
From The Runners Guide to Tokyo:
Most non armory runners will get their introduction to Tokyo through the Yakuza (and some of those join up with one of the families). Most of the wetwork for runners will come from the Yakuza, and often involves their own internal warring.
Also, remember to watch out for their tattoos. They channel a potent form of magic, and grow larger and more complex, the more powerful the individual Yakuza becomes.