Luis Cardinal was a Nicaraguan former mercenary and assassin-turned-operative for the Syndicate, who worked on a number of operations with Alex Krycek, while under the command of the Cigarette Smoking Man. He was partly responsible for the murder of Melissa Scully. (TXF: "The Blessing Way", "Paper Clip", "Piper Maru", "Apocrypha")
Luis Cardinal worked alongside the Central Intelligence Agency in Nicaragua and numerous other Southern and Central American countries, acting as a mercenary and bounty hunter. He received military and operational training at the School of the Americas.
He was hired by the Cigarette Smoking Man, though he doesn't seem to have had direct links to the Syndicate.
The Digital Tape
Cardinal and Krycek were tasked with the assassination of Dana Scully, ordered by the Syndicate, and hid in her house in the early hours of the morning, awaiting her return. However, Melissa, Scully's sister, arrived first and Cardinal shot her instead. After Krycek identified her as the wrong person, the pair fled the scene. (TXF: "The Blessing Way")
Following this botched operation, Cardinal, Krycek and a third man were ordered to recover the digital tape from Assistant Director Skinner. They ambushed him in the stairwell of the hospital where Melissa was being kept, beating him and stealing the tape before they escaped.
With the tape in their possession, the three stopped at a petrol station, where Cardinal asked whether Krycek wanted to come inside the shop there with them. When Krycek declined to accompany them, Cardinal showed evident surprise before exiting, tipping off Krycek that something was amiss and ultimately warning him of the car bomb that he subsequently escaped. (TXF: "Paper Clip")
Cardinal's next assignment came when Skinner continued to persist with the investigation into Melissa's death. In an attempt to stop the potential findings of the murder, a group of G-Men confronted Skinner in his local cafe, telling him to desist with his inquiries. When Skinner angrily refused, Cardinal was sent in to kill him. Going into the same cafe and staging an altercation with a waitress, Cardinal waited for Skinner to move to her defense before shooting him, making it appear to be a random murder. However, Skinner survived, and recognized Cardinal's face from their fight in the hospital stairwell. (TXF: "Piper Maru")
Cardinal was later called upon by the CSM to provide security, when he dealt with the black oil-infected Krycek.
Having failed in his previous attempt at the mission, Cardinal was ordered to fulfill his orders and an ambush for Skinner was set up while he was being moved to a different hospital. However, the plan had not taken into account that Scully would be in the ambulance with Skinner. When Cardinal attempted to storm the doors, he was shot and winged by Scully before doing any damage, and was then pursued by her. After a chase, Scully finally caught up with him and angrily demanded to know whether he had been responsible for killing her sister. Cardinal confirmed that he had been present at Melissa's death and that he could give information to Agent Scully. Cardinal was arrested and taken into custody, then informed Scully of Krycek's destination of North Dakota. Before he could testify, he died in prison after being hanged in his cell, presumably silenced by the Syndicate. (TXF: "Apocrypha")