Yuki Matsumuro was head of a japanese criminal clan, the Bakuto.
When the Baku escaped custody, Yuki was concerned that he would challenge her for control of the Bakuto and made a covert deal with Bombshell : Yuki would lure the Baku out of hiding by proposing a merger of their two clans, and Heroes Alliance would take him back into custody.
Unfortunately for Yuki (and unbeknownst to the rest of Heroes Alliance), Bombshell was responsible for the Baku's initial escape and had been using him to draw out both leaders and their clans so they could be captured. (As a bonus, she was also able to test new recruit, Amalgam's abilities, and to increase ties with local Japanese hero, the Bujin.
Yuki has no powers but she is extremely wealthy, head of a ninja clan - the Bakuto - and appears to be a highly skilled swordswoman.