From Tekken Zaibatsu Wiki
After defeating her sister, Anna, Nina Williams was without an immediate goal to attain. Jin Kazama, recently appointed the leader of the Mishima Zaibatsu, recruited her as a member of his special forces. As someone who wasn?t selective about her clients, Nina engaged in covert demolition operations around the world as part of the Tekken Force.
As the conflict between the Mishima Zaibatsu and the G Corporation intensified, the Mishima Zaibatsu announced The King of Iron Fist Tournament 6. Nina decided to enter the Tournament to eliminate anyone who poses a threat to Jin Kazama.