Kai Len. "Now we're going to have some fun"
Kai Leng is an assassin working for Cerberus. He is a former lieutenant and N7 agent, demoted and sentenced to twenty years for killing a krogan in a bar fight on the Citadel during his leave. Kai Leng was born on Earth and has Russian-Chinese heritage.
With his skill and xenophobia, Leng caught the attention of Cerberus. The Illusive Man orchestrated his escape from prison and used Leng as an agent for ten years. Leng became the Illusive Man's most loyal aide, spy, and assassin. Among his implants, it appears that he has modules used by Cerberus Phantoms.
2172 — Terminus Systems. Patrol and reconnaissance.
2173 — Exodus Cluster. Patrol and reconnaissance. (Received a reprimand for stripping decorations from the bodies of fallen enemy officers; punishment mitigated due to exemplary service record.)
2174 — Yafen System. Assault on Entyion (attached to a Medal of Valor from the Systems Alliance).
2175 — Hawk's Nest, tactical reconnaissance (enrolled in the Interplanetary Military Academy).
2176 — Appointed to N7 from the Interplanetary Military Academy.
2176 — Dishonorably discharged and placed in custody on charges of first-degree murder.
Mass Effect: Retribution
When Cerberus confirmed that Paul Grayson was on Omega, Kai Leng was sent to track him down and capture him. Leng, along with a team of six Cerberus operatives, successfully completed the mission, during which he personally killed Liselle - daughter of Aria T'Loak and Grayson's love interest. He was ordered to keep an eye on Grayson during the Reaper nanotechnology experiments. It was Leng who suggested dosing Paul with red sand to lower his resistance to Reaper technologies.
The Illusive Man ordered Leng to kill Grayson if the experiment became too dangerous or if he slipped out of control. But when a Turian strike team stormed the research facility where Grayson was located, Leng chose to save the Illusive Man instead of killing his charge, which saved Paul's life. After the attack, the Illusive Man sent Leng to Aria to convince her to assist Cerberus in finding Grayson, as well as to keep tabs on the asari.
After Grayson's escape from Omega, Kai Leng offered David Anderson and Kali Sanders — who were also searching for Grayson — a ride off Omega in exchange for their help in eliminating the Reaper-possessed monster that used to be Grayson.
Knowing that Leng was a Cerberus agent, Anderson and Kali attacked him, but he got the better of them and once again asked for help, remembering that Kali was key to finding Grayson. They reluctantly agreed, but at Leng's shuttle, they ran into a group of Turian guards. Anderson managed to knock out the distracted Leng and convinced the guards to let him take the assassin's body with him. Tying Leng's body to the pilot's seat, they set course for the John Grissom Academy, home to Project Ascension, where the Reapers had sent Grayson.
Upon arriving at the Grissom Academy, Anderson and Kali left the shuttle to protect the biotic students, leaving Leng tied to the seat. After breaking free from his bindings, Leng pursued Kali and Anderson into the academy. Encountering a biotic student named Nick, he persuaded him to side with him, as he needed Nick's biotics as a weapon.
Leng wanted to incapacitate Grayson using Nick's biotics, but when the attempt failed, he tried to steal Paul's weapon. He was unsuccessful; moreover, Leng almost perished but was saved by Anderson, who shot the Reaper-controlled Grayson with a shotgun. Anderson was faced with the choice of helping the injured Nick or letting Leng escape. He chose the life of the student, though he shot Leng in both legs to slow him down. Despite his injuries, Leng managed to return to the shuttle and flee.
Mass Effect: Deception
Information from the first edition of Mass Effect: Deception, deemed inaccurate by Bioware due to discrepancies with earlier lore of the Mass Effect universe.
By the events of Mass Effect: Deception, Kai Leng's wounds have healed, but he uses a cane when walking. He can still fight as well as before, but the strain causes intense pain in his legs. When blueprints for a bioweapon specifically designed to attack the Illusive Man were stolen, Leng finds them at an auction in the city of Throndu on the batariian homeworld of Harshan. Leng steals the blueprints and escapes. Later, Leng travels to an uninhabited planet in the Crescent Nebula and receives new orders from the Illusive Man: extract Paul Grayson's body from the Citadel and monitor the activities of Kali Sanders and David Anderson.
Mass Effect 3
In Mass Effect 3, Kai Leng is one of the agents sent by Cerberus to confront Shepard. Like Cerberus Phantoms, he is equipped with a device that projects a kinetic barrier, a cloaking cape, and a melee sword.
Leng was supposed to kill the salarian Counselor while Cerberus, with Udina's assistance, was attempting to seize control of the Citadel.
Here, two options arise:
He fails to execute his plan as Shepard appears with the team and Major Kirrahe/Teyn Kryos. After a brief scuffle, Leng manages to escape, having wounded Teyn/Kirrahe.
Leng kills the Counselor if both Teyn and Kirrahe were killed in the first games.
Shepard gives chase after Leng, who now must reach the other members of the Council. Leng disables Shepard's X3M, but he still manages to reach the Council first. Ultimately, Leng fails the mission and is forced to retreat. Before leaving the Citadel, Leng released an AI into the electronics and erased all records of himself.
The next confrontation between Leng and Shepard happens on Thessia, the Reaper-occupied world, where the captain arrives to obtain crucial information on the Catalyst needed to activate the Crucible. Leng arrives on the planet for the same reason. When Leng begins to lose, he collapses the floor of the temple, grabs the data, and flies away from the planet.
Shepard falls into the resulting chasm but manages to grab onto something and climb back up. Shortly after, a message from Kai Leng titled "Evacuate Thessia" appears on the captain's terminal, disguised as a message from asari command.
Samantha Traynor managed to track Leng to Horizon, where Henry Lawson was conducting experiments on refugees. At the same time, a former Cerberus operative — Miranda Lawson, is also there, searching for her sister. The Illusive Man ordered Leng to eliminate his former subordinate if she gets in his way. Kai wounds Miranda but leaves her alone. He successfully completes his mission, but he doesn't know that Miranda marked him so that Shepard could strike at Cerberus.
The final confrontation between Shepard and Leng occurs at the Kronos Station — the Cerberus base, when the assassin faces the captain on the station's observation deck. In this battle, Leng could not win.
Believing the opponent dead, Shepard settles into the Illusive Man's chair. Leng, despite his severe wounds, rises, draws his sword, and silently approaches to kill the captain from behind. At the critical moment, Shepard notices him and halts the attack. If you managed to press the button when the renegade symbol appeared, then before killing Leng, Shepard breaks his sword and then delivers the fatal strike with a unique blade. Thus ends the life of the greatest assassin of Cerberus.
Trivia:
In Mass Effect 3, Kai Leng's body is filled with cybernetic implants, making him even deadlier than in the books.
Leng's weapon is based on the Japanese "Ninjato" — a straight (and often shortened) variant of the samurai's katana. This weapon suits Leng's style, aiming to deliver a quick fatal strike from close range.
Leng has Russian-Chinese roots, but the Chinese part dominates.
Kai Leng is voiced by Troy Baker.
Leng has a tattoo on his neck in the shape of an Ouroboros — a serpent eating its tail.
Sources:
Thanks to Surt for proofreading.