Kai Leng

Kai Leng was originally an N7 Alliance marine lieutenant known for having anti-alien attitudes. He is described as being Earth-born with features that show his dominant Chinese heritage with a slight hint of some distant Slavic ancestry in his jaw line. During his service, he was arrested for the murder of a krogan in a bar on the Citadel using only a knife, and sentenced to twenty years in an Alliance prison. His prowess in combat and xenophobic nature drew the attention of the Illusive Man, and Cerberus helped liberate Leng from prison. The Illusive Man used Leng as his best wet-work operative and assassin for over a decade, realising he was not only ruthless but discreet and methodical.

Mass Effect: Retribution
When Cerberus confirmed Paul Grayson was on Omega, Leng was charged with hunting down and capturing him. Leng and a squad of six other Cerberus operatives captured Grayson. During the mission Leng personally killed Liselle, the daughter of Aria T'Loak and Grayson's lover. He was then ordered to oversee Grayson during the Reaper nanotechnology experiment. It was Leng's idea to dose Grayson with red sand to lower his resistance to the Reaper technology.

The Illusive Man ordered Leng to exterminate Grayson when the experiment became too dangerous to continue, or if he got out of control. However, when a turian strike team raided the research facility that housed Grayson, Leng chose to rescue the Illusive Man rather than kill Grayson, meaning Grayson was rescued by the turians. After the assault, the Illusive Man sent Leng to Aria as Cerberus' representative, both to negotiate for her assistance in finding Grayson, and to keep an eye on her.

After Grayson escaped Omega, Kai Leng offered David Anderson and Kahlee Sanders - who were both looking for Grayson - a ride off Omega, in exchange for their help in taking out the Reaper-possessed monster Grayson had become. Knowing he was a Cerberus agent, Anderson and Kahlee attacked Leng, but he got the upper hand and asked again for their help, knowing Kahlee was the key to finding Grayson. They reluctantly accepted; however, when confronted with turian security guards near Leng's shuttle, they took their chance. After distracting Leng, Anderson tasered him, then convinced the guards to let him take Leng's limp body. Leng was then tied to the copilot chair with rope from the shuttle's med kit. During the journey, Kahlee realised the Reapers were sending Grayson to the Jon Grissom Academy, home of the Ascension Project.

Upon arrival at Grissom Academy, both Anderson and Sanders left the shuttle to protect the biotic pupils at the Project, leaving Leng tightly bound to the chair. After a struggle with the rope that left him bleeding, Leng managed to free himself and get into the Academy. He later encountered a gifted biotic student named Nick, and needing Nick's biotics in lieu of a weapon, managed to persuade Nick that he was on his side.

On his way through the Academy, Leng heard Grayson in the distance, prompting both he and Nick to hide in a nearby empty room. Leng intended to ambush Grayson using Nick's biotics, but when the attempt failed, Leng tried to steal Grayson's weapon. He failed and was almost killed, only to be saved by Anderson, who killed the Reaper-controlled Grayson with a shotgun.

Anderson refused to simply let Leng escape; however, Nick had been shot and was badly hurt. Anderson had to choose between saving Nick and letting Leng go, eventually choosing to help Nick - though not before wounding Leng in each leg to try and slow him down. Despite his injuries, Leng still managed to get back to the shuttle and escape. He was able to report to the Illusive Man and assured him that, despite the failures of his mission - he was still an asset to Cerberus.

Mass Effect: Deception
By the time of Mass Effect: Deception, Kai Leng's wounds have healed but he uses a cane when walking. He can still fight much as he used to, but the exertion can cause considerable pain in his legs. When a design for a DNA-specific bio-weapon tailored to attack the Illusive Man is stolen, Leng tracks it to an auction house in the city of Thondu on the batarian homeworld of Khar'shan. Leng steals the design and escapes. Later, Leng arrives on an uninhabited world in the Crescent Nebula and receives new orders from the Illusive Man: retrieve Paul Grayson's body from the Citadel and monitor the activities of David Anderson and Kahlee Sanders.

Mass Effect 3
In Mass Effect 3, Kai Leng is amongst the forces that Cerberus sends against Commander Shepard.

Trivia

 * In Cantonese, the name "Kai" means "following intimately" or "unceasing", possibly alluding to his hunt for Grayson.