Liu Yantsui was the adoptive older brother of Liu Fei Long and the biological son of Liu Talen. He had a twisted obsession with his younger brother and feared that the more talented younger one would take his heritage and position from him. After killing his father, Yantsui left Hong Kong and went to Taiwan, eventually becoming head of a Taiwanese mafia under a false identity.
At some time around the Naked Truth arc he got into contact with Mikhail Arbatov with whom he wanted to do business. Instead of building up a furitious business-connection with the Taiwanese mafia of Yantsui, Mikhail provided the other man with drugs and got him hooked on them.
Around the same time the information that Yantsui is still alive reaches Fei Long. While his brother's survival is a dire threat to his life and position, the laws of the Triad forbid Fei Long to murder a member of his own family. Therefore he travels to Taiwan, meeting with Yoh, who had fled there after the occurances on the Casino Ship. Fei Long asks Yoh to kill Yantsui for him, to which Yoh agrees.
Meanwhile however Yantsui learned about Fei Long's arrival in Taiwan. He takes Tao - who had followed Fei Long secretly out of curiousity - hostage, forcing his younger brother to come to his hideout alone, where he abuses and rapes his brother, stopped only before the final act when Yoh comes to save Tao and Fei Long.
In the end, Yoh executes Yantsui - yet not before Tao learns that Yantsui is indeed his biological father.