Michael Thorton meets Scarlet Lake on a plane not long after Saudi Arabia. Throughout the game, Thorton can choose to sell Scarlet information via e-mail for reputation points and a little bit of money.
It is later revealed that Scarlet is actually a world class assassin who uses her job as a photojournalist as a cover. She is the one who attempts to assassinate the Taiwanese President Ronald Sung. Whether she succeeds or fails depends on the player's actions. Thornton can later kill her for betraying him or ally with her if she loves him enough to join his cause in the end.
While Scarlet is pretending to be a globe-trotting photographer she tells Thorton that she has followed in the career path of her mother, suggesting her mother was also a photographer. Whether this was just part of her cover, or her mother was actually an assassin masquerading as a photographer like herself is never explained further.
Depending on your reputation with her during the game (Having a "Trusted" reputation status), she is one out of four possible romance options.