Kazu Tamazaki was a modern day samurai responsible for killing many. He used the pretence of being the second in command of a man named Roman Vadik - who did not exist at all - as a cover.
Before Season 4
Tamazaki went to the Hazunaga Asian Museum in London and was arrested for breaking in. His break-in to the Shintaro sword exhibit resulted in the death of two MPs and one of Tamazaki's own men. His was unsuccessful in his aim to steal the Shintaro sword, the most famous Samurai sword outside of Japan (Authorized Personnel Only, Part 1).
Tamazaki met with Michael Vaughn on the train from Belarus to Latvia, thinking that he was Yuri Komarov and would sell him the dangerous isotope Aurine-12. The transaction went ahead but Tamazaki became suspicious of Vaughn and eventually convinced that he was not Komarov. He left the train without paying Vaughn and told his men to kill Vaughn. Vaughn overpowered them but Tamazaki disappeared and APO had no trace of where he had gone (Authorized Personnel Only, Part 1).
Arvin Sloane came up with the only viable solution to get Tamazaki's attention; steal the Shintaro sword he so desired. Once they acquired it, APO wanted for Tamazaki to hear that it had been stolen. Meanwhile Jack Bristow learnt from a contact that Roman Vadik, Tamazaki's supposed boss, did not exist and was made up by Tamazaki as a cover for himself. Tamazaki took APO's bait and agreed to a meet in Rio de Janeiro, demanding that Sydney, as the woman who stole the sword, be the one to deliver it. She did so but Tamazaki tricked her and kidnapped her and the sword.
He strapped her to a chair and demanded to know why she did not stick to the deal. Sydney was confused about what he meant. When she still did not understand what Tamazaki was talking about when he refered to "the hit", he began to torture her by drowning her in a gas mask full of water. He eventually explained to her that he had a contract on her life but his client was killed the day before the hit was supposed to take place. She said that she knew nothing about the hit, and Tamazaki believed her but refused to let her leave. He said he would take the sword and kill her.
Nadia, after learning from Vaughn that Sydney was in trouble, travelled to Rio and told APO about the Catedral Metropolitana where Tamazaki used to run operations. Just before Tamazaki drowned Sydney, Nadia entered (after having tranquillized a guard) and saved her. Tamazaki fought Nadia and knocked her out, but Jack and Vaughn arrived and Tamazaki ran. Sydney pursued him and managed to kill him with the sword he used against her. As he died, Tamazaki told Sydney that it was her mother that hired him to kill her (Authorized Personnel Only, Part 2).