On the surface, Katherine Zidler is a beautiful southern socialite with a penchant for gambling, in Sydney for the poker tournament with her friend Tatya Ivanov and her boyfriend Ian Crosby. She was beaten by Rembrandt during the poker tournament in Sydney, and as she congratulated him on his victory, he got the distinct impression she was more than what she seemed.
This turned out to be entirely true; she, along with the other members of Tranquility-6 engaged in a brutal fight on Davenport's yacht, escaping with yet another fake briefcase and leaving their dead behind. Although they attempted to circle back to retrieve the true briefcase, Ace escaped with it.
Although foiled this time, such a ruthless, seasoned operative isn't going anywhere soon. Whoever Katherine Zidler really is, odds are the Team hasn't seen the last of her.