Siobhan Sadler was a mercenary.
She was part of a group who were hired to steal the Diadem. They succeeded, but Ace pursued them and fired an EMP at their ship. This knocked out the safety seals on the case the Diadem was in and let it exert its influence on Sadler. It eventually gained complete control over her, but she died while being chased by others after the Diadem. (PROSE: Legacy)