Kai Leng

Kai Leng is the main antagonist in Mass Effect: Retribution and Mass Effect: Deception, and the secondary antagonist and final boss in Mass Effect 3. He is a Cerberus assassin and a former N7 operative. Like the vast majority of Cerberus, he is racist towards aliens.

He is voiced by Troy Baker

History
He joined Cerberus after being dishonorably discharged from the Alliance and imprisoned for murdering a krogan at a bar on the Citadel with a standard issue service blade. Cerberus helped him escape prison, and Leng soon became one of the Illusive Man's top operatives.

Mass Effect: Retribution
Kai Leng is sent to capture a former member of Cerberus called Paul Grayson and later to murder him when Reaper tech indoctrinated him and made him too dangerous to live. In Deception, he is tasked with retrieving Paul Grayson's body from the Citadel.

Mass Effect 3
He is amongst the Cerberus forces attacking the Citadel during the the attempted coup. He encounters Commander Shepard whilst attempting to assassinate the salarian councillor. He also appears later on Thessia where he takes the Prothean VI containing data on the Catalyst from Shepard. Shepard later traces Leng to Sanctuary, a refugee centre on Horizon, which turns out to be a Cerberus facility used to study Reaper indoctrination. The Illusive Man had ordered Leng to Sanctuary in order to retireve the research data from the facility, but by the time Shepard arrives he has already left.

Leng makes his final appearance at Cerberus Headquarters, where he is finally killed by Shepard and the VI is recovered.

Trivia

 * He's supposed to be a foil of Shepard.
 * While being primarily Chinese in heritage, he is also of Slavic descent.
 * In early concept he was supposed to be a mechanized soldier, later his appearance was designed to look more like an assassin Thane Krios, they even fight once.
 * In cut scenes he becomes invisible and summons force field, but not in the gameplay despite he has implants of Cerberus Phantom.