Kai Leng

"Cerberus thanks you for all of your hard work."

- Kai Leng to Commander Shepard

Kai Leng is an assassin 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. Kai Leng is amongst the forces that Cerberus sends against Commander Shepard.

Leng, along with a sizable contingent of Cerberus forces, were 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 interfered. After a brief skirmish, Leng managed to escape after stabbing Krios/Kirrahe. If both Thane and the major had previously died, Leng will successfully assassinate the councilor.

In a race to get to the Council, Leng disabled Shepard's X3M, but ultimately failed his mission and was forced to retreat as C-Sec began to regroup. As he escaped, Leng reported his failure to the Illusive Man and promised 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 was later sent to the Temple of Athame on Thessia, while being invaded by the Reapers, where he killed a group of scientists waiting for Shepard. Leng remained hidden as the Commander discovered a hidden Prothean Beacon, awakening a Prothean VI named Vendetta, who held information on the final Crucible piece known as the Catalyst. Leng then showed him, at which Vendetta shut down upon detecting that he was indoctrinated. Leng requested the Commander's attention and released a small drone that projected a hologram of the Illusive Man. After the Commander and the Illusive Man exchange words, the latter ordered Leng to take the VI. The Cerberus assassin then engaged Shepard in combat but was 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 he claims the data.

Samantha Traynor was able to track Leng to Horizon, where Henry Lawson was conducting experiments on refugees. At the same time, ex-Cerberus operative Miranda Lawson was there, looking for her sister. The Illusive Man reminded Leng to get the data but to kill Miranda only if she got in the way. Leng wounded Miranda when she confronted him, but otherwise left her alone. He managed to complete his primary objective and abandoned the Lawsons to die, but was unaware that Miranda had tagged him so Shepard could strike back at Cerberus.

Shepard's final confrontation with Leng takes place during the Alliance raid on Cronos Station, when the assassin confronts the Commander on the observation deck of the station. 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.

(From the Mass Effect Wikia)