Kai Leng

Kai Leng is a douche canoe working for the human-survivalist organization Cerberus. Leng was formerly a lieutenant and N7 marine in the Systems Alliance known for having anti-alien attitudes. He is described as being Earth-born with features that show his dominant Chinese heritage.

Dossier
While little is known about Kai Leng's early life, Alliance records show that he enlisted in the Alliance Military on November 14, 2172 at the age of 16, although his credentials were faked at the time. Leng's prowess in combat and xenophobic nature drew the attention of the Illusive Man, and Cerberus helped liberate Leng from prison after he was incarcerated for murder. 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. Leng became the Illusive Man's most trusted agent, working as an infiltrator and assassin. His cybernetic modifications appear to include Cerberus's Phantom-class implants.

Tours of Duty

 * 2172 Terminus Systems Patrol and reconnaissance
 * 2173 Exodus Cluster Patrol and reconnaissance (reprimanded for taking medals from dead enemy officers; sentence was reduced in light of previous, exemplary service record)
 * 2174 Raid of Entiyon (recommended for Systems Alliance Medal of Valor)
 * 2175 Kite's Nest Tactical reconnaissance (accepted into Interplanetary Combatives Academy)
 * 2176 Received N7 designation from Interplanetary Combatives Academy.
 * 2176 Dishonorably discharged and imprisoned for 20 years in an Alliance prison after charge of first-degree murder (while on leave, murdered a krogan in a bar on the Citadel using a standard-issue service blade).

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 capture Grayson. During the mission Leng personally kills Liselle, the daughter of Aria T'Loak and Grayson's lover. He is 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 orders Leng to exterminate Grayson if the experiment becomes too dangerous to continue, or if he got out of control. However, when a turian strike team raids the research facility that houses Grayson, Leng chooses to rescue the Illusive Man rather than kill Grayson, meaning Grayson is rescued by the turians. After the assault, the Illusive Man sends Leng to Aria as Cerberus' representative, both to negotiate for her assistance in finding Grayson, and to keep an eye on her.

After bringing the proposals of the Illusive Man to Aria, Leng remains on Omega, closely monitoring Aria's movements and waiting for Grayson to show up. After Grayson arrives on Omega, lured by a distress call from Kahlee Sanders, Leng follows Grayson, but doesn't engage him because of his cybernetic enhancements. He stays out of the way as Grayson tears through Aria's troops and flees to the spaceport.

After Grayson escapes Omega, Kai Leng offers David Anderson and Kahlee Sanders - who are 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 is a Cerberus agent, Anderson and Kahlee attack Leng, but he gets the upper hand and asks again for their help, knowing Kahlee is the key to finding Grayson. They reluctantly accept; however, when confronted with turian security guards near Leng's shuttle, they take their chance. After distracting Leng, Anderson tasers him, then convinces the guards to let him take Leng's limp body. Leng is then tied to the copilot chair with rope from the shuttle's med kit. During the journey, Kahlee realises the Reapers are sending Grayson to the Jon Grissom Academy, home of the Ascension Project.

Upon arrival at Grissom Academy, both Anderson and Sanders leave 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 manages to free himself and get into the Academy. He later encounters a gifted biotic student named Nick, and needing Nick's biotics in lieu of a weapon, manages to persuade Nick that he was on his side.

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

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

Mass Effect: Deception
''This section features material from the first edition of Mass Effect: Deception, which BioWare has acknowledged contains discrepancies with other Mass Effect lore. Once revised editions of the book are released, this section will be updated.''

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
Kai Leng is amongst the forces that Cerberus sends against Commander Shepard. Like Cerberus Phantoms, he is equipped with a palm device that projects a kinetic barrier, a projectile weapon in his right gauntlet, a tactical cloak and a sword for close-range combat.

Leng and a sizable contingent of Cerberus forces are deployed during Councilor Udina's attempted coup against the Citadel Council. He was close to assassinating the salarian councilor when Shepard and either Major Kirrahe or Thane Krios interferes. After a brief skirmish, Leng manages to escape after stabbing Krios/Kirrahe. If both Thane and the major previously died, Leng will successfully assassinate the councilor.

In a race to get to the Council, Leng disables Shepard's X3M, but ultimately fails his mission and is forced to retreat as C-Sec begins to regroup. As he escapes, Leng reports his failure to the Illusive Man and promises to never let Shepard get the better of him again. It's revealed that Leng released a VI into the Citadel mainframe that erased all footage of himself. Bailey presumes that Leng didn't believe that Udina's plan would succeed.

Leng is later sent to the Temple of Athame on Thessia, while being invaded by the Reapers, where he kills a group of scientists waiting for Shepard. Leng remains hidden as the Commander discovers a hidden Prothean Beacon, awakening a Prothean VI named Vendetta, who holds information on the final Crucible piece known as the Catalyst. Leng then reveals himself, at which point Vendetta shuts down upon detecting that he is indoctrinated. Leng requests the Commander's attention and releases a small drone that projects a hologram of the Illusive Man. After the Commander and the Illusive Man exchange words, the latter orders Leng to take the VI. The Cerberus assassin then engages Shepard in combat but is unable to stop Shepard, so he instructs his gunship to level the temple. The floor caves in, leaving Shepard alive but unable to act as Leng claims the data and escapes. After the mission, Leng sends a taunting message to Shepard titled "Evacuating Thessia", disguised as asari military command:

Samantha Traynor is able to track Leng to Horizon, where Henry Lawson is conducting experiments on refugees. At the same time, ex-Cerberus operative Miranda Lawson is there, looking for her sister. The Illusive Man reminds Leng to get the data but to kill Miranda only if she got in the way. Leng wounds Miranda when she confronts him, but otherwise leaves her alone. He manages to complete his primary objective and abandons the Lawsons to die, but is unaware that Miranda has tagged him so Shepard can strike back at Cerberus.

During the Alliance assault on Cronos Station, Shepard discovers video logs, detailing Leng's frustration with the Illusive Man's refusal to have the Commander killed. Eventually, the assassin reveals himself in the Illusive Man's office, interrupting the Commander's attempt to warn Admiral Hackett about the Catalyst being the Citadel. Leng then fights Shepard (along with a small squad consisting of seven Assault Troopers, two Phantoms, and a Nemesis). Despite Leng's lethal skills, he's unable to overwhelm Shepard. With his shielding depleted, he soon succumbs to attacks from Shepard's squad.

However, despite his heavy wounds, Leng retrieves his blade and silently approaches Shepard to kill the Commander from behind. At the critical moment, Shepard detects him and counters his attack. The Commander can either break Leng's sword while stabbing him with an omni-blade, or dodge the attack and stab him with the omni-blade. With either outcome, Shepard ends the life of Cerberus' greatest assassin.

Trivia

 * According to The Art of Mass Effect 3, early concepts of Kai Leng gave him metal legs and hard armor, but this evolved into a stealthier appearance with a coat reminiscent of the one worn by Thane Krios. His face and body kept a few obvious cybernetic implants to imply that he had been modified since the events in the books to become even deadlier.
 * According to the Mass Effect novels, Kai has both Chinese and Russian/Slavic heritage, with the Chinese being the dominant of the two.
 * The weapon wielded by Leng appears to be based on a Japanese "ninjato"- a straight (and often shorter) variant of the samurai's katana.