Kai Leng

Kai Leng was an N7 ranked marine in the Alliance but was arrested for the murder of a krogan on the Citadel with only his service knife. Cerberus helped liberate him from an Alliance prison and then The Illusive Man used Leng as his best wet-work assassin. Leng was charged with hunting down and capturing Paul Grayson when his identity was confirmed on Omega. Kai and a squad of six other Cerberus operatives captured Grayson and killed Liselle. Kai Leng was ordered to oversee Grayson during the Reaper nanotechnology experiment and exterminate him when he got out of control. When a turian raid ended up assaulting the research facility that harbored Grayson, Kai chose to rescue The Illusive Man rather then kill Grayson. After the assault The Illusive Man sent Kai as a representative of Cerberus to strike a deal and keep an eye on Aria T'Loak.