Kai Leng is an ex-N7 operative - discharged and jailed after murdering a Krogan in a bar with nothing but a knife. While incarcerated, he was recruited by The Illusive Man and has served as his "wet work" operative under Cerberus since. Most notably, among the names of people murdered by him is a daughter of the pirate queen Aria T'loak.
Mass Effect 3
Leng is first encountered during the Cerberus invasion on the Citadel. His main directive is the assassination of the Salarian councilor. If Thane joined the player's squad in Mass Effect 2 and survived the suicide mission - and the player speaks to him on the Citadel before the coup - he will arrive to successfully protect the councilor; albeit, at the cost of his own life. If Thane is unavailable, Major Kirrahe will take his place. If Kirrahe is unavailable, Leng will successfully assassinate the Salarian councilor.
Nevertheless, through the aforementioned intervention from the chosen character and/or Shepard's actions, Leng fails to kill the rest of the Citadel council and is forced to retreat from the station.
Leng is encountered once again in the Temple of Athame on Thessia. Having executed a group of scientists awaiting the Commander, he lies in wait for their arrival. When the Commander proceeds to discover a hidden Prothean beacon, along with its associated VI, 'Vendetta', who holds information on the final piece of the Crucible known as 'The Catalyst', Leng makes himself known. In response, Vendetta immediately shuts itself down, aware that Leng has been indoctrinated.
Leng deploys a drone to project a hologram of The Illusive Man, who speaks with Shepard; however, unable to come to an agreement, the former orders Leng to seize Vendetta. Leng attempts to do so, engaging in combat with the Commander and their team but is unable to defeat them. He commands his nearby gunship to destroy the temple, successfully dismantling the floor and allowing Leng to steal the data from his incapacitated foes.
Following Samantha Traynor's efforts to track him, Leng is located - and next encountered - on Horizon. His goal is to secure data attained by Henry Lawson through the latter's experiments on refugees. Also present is Miranda Lawson, searching for her sister. Upon encountering Leng, Miranda is wounded but not eliminated (as The Illusive Man only deems her a necessary elimination if she gets in the way.) Leng successfully acquires the data and exfiltrates, leaving the Lawsons to die - though not before Miranda is able to tag him without his knowledge. This allows Shepard to hunt Leng - and, by extension, Cerberus itself - to a final confrontation at Cerberus's Headquarters on Cronos Station.
During the Alliance's assault on Cronos Station - and, by extension, the Commander's time there - the latter learns of Leng's frustration with The Illusive Man's restraint in seeing the Commander's elimination at Leng's hand. Later, when the Commander attempts to warn Admiral Hackett about the Catalyst being the Citadel via The Illusive Man's office holographic communicator, Leng appears. He brings elite Cerberus troops to support an all-out assault against the Commander's squad, but is unable to defeat them.
Now heavily-wounded, Leng attempts a final, stealthy assassination against his new nemesis...only to be countered by Shepard in the last moment. Stunned by this reversal of fortune, Leng may have his sword broken or avoided - only to be followed, either way, with a final, fatal Omni-Blade attack from the Commander. His death is dedicated to whoever Leng has killed - 'That was for (characters), you son-of-a-bitch' - or if none were involved, to an end of his dirty tricks ('No gunship this time, you son-of-a-bitch'.)