Lau Kin Ming is a central character in Infernal Affairs. He appears in all three films.
Lau Kin Ming is an undercover agent for the Triads, sent by Triad boss Sam to leak information and Wong's next actions to gain an upperhand.
In 1991, Sam's wife Mary ordered the assassination of Ngai Kwun, the head of the Triads in Hong Kong. Having paid off Kwun's bodyguards, Lau succesfully carried out the assassination and this action ignites friction between the Ngai family and the Hong Kong Royal Police.
Through the years, Lau slowly climbs up the ranks of the police force through tips off from Sam himself, and firmly establishes himself in a spot for future promotions. His crush on Sam's wife, Mary was a deciding factor in choosing his life as a Triad undercover. When Mary rejects his advances, a dejected and angry Lau seeked revenge. In 1995, Lau tipped off the Ngai family on Mary's whereabouts, resulting in her death. Lau was present during the time when Ngai Wing Hau holds his boss Sam, hostage at gunpoint. He helped Sam escape police witness protection, allowing Sam to infuriate Hau and incriminate him in the presence of the police.
During the 1997 handover of Hong Kong, Lau met his future fiancee also named Mary.
In 2002, Lau's actions were pivotal in Sam's cocaine operations. He constantly relays information secretly to Sam on Wong's next actions, and this led to a back and forth stalemate on both sides. Sam grows impatient and paranoid, and held evidence of Lau's dealings with him in recordings. As a result of Wong's death, Lau ponders on his identity, and ultimately seeked to turn over a new leaf. He contacted Wong's undercover, Wing-Yan in a final attempt to nail Sam. Lau manages to corner Sam in a isolated area before killing him.
However, Wing-Yan discovers that Lau is the undercover for Sam and is determined to arrest him and bring him to justice despite pleas from Lau to give him a chance. Wing-Yan was killed by fellow colleague Billy, who revealed himself as another undercover of Sam's. As they both head down to face the police, Lau kills Billy in the lift and emerges, identifying himself as a police and therefore keeping his triad roots a secret.
Following the events after Wing-Yan and Billy's death, Lau is put under investigation. He gave his skewed version of the event, claiming he contacted Yan and found him held hostage by Billy. His eyewitness account states that Yan tried to disarm Billy in a futile effort, and Lau shot him with Billy accidentally damaging the lift's CCTV cameras.
In 2003, the case was closed and Lau's killing of Billy was deemed fair. He was temporary placed in menial office work, before being transferred back to Internal Affairs. During this time, he discovered a secret between Yeung and Shen. Trying to cover his tracks once and for all, Lau attempts to frame Yeung as the final mole placed by Sam.
During this period, Lau suffers severe mental breakdown as a result of his guilt. He sees himself as Yan, and his split personality disorder effectively took over his fragile state of mind. Lau leaks out his real identity unknowingly to Doctor Lee under hypnosis, when she witnessed Lau behaving irratically at the hospital after a car accident.
Lau proceeded to confront and arrest Yeung with tapes he found in his office. His mental state affects his judgement and he plays the tape to everyone on the scene, that incriminates himself instead. Lau attempted suicide after killing Yeung, surviving the ordeal but can no longer properly function like a normal person due to his wound.