Tyler Brennen was an arms dealer who operated out of Miami.
Brennen has a young daughter named Annabelle who is enrolled in a private school in Switzerland for $40,000 per semester. He supports her education as well as his private life by dealing in guns and weapons of mass destruction across international waters.
In the beginning, Brennen was a spy just like Michael except he later became disillusioned with the whole spying career but the reasons for this are unknown. Brennen went rogue and soon got burned as a result. This prompted Brennen to become an arms dealer, a career he has maintained until his death.
Michael first encountered Tyler Brennen when Brennen forced Samantha to steal a guidance chip from AFC Industries in exchange for her son's life who he had kidnapped and was holding hostage. After failing to make any profit from it, thanks to Sam Axe firing on his clients with his Beretta, Brennen had no choice but to hand back the chip to Michael and disappear from Miami.
A year or so later, Brennen re-emerged yet again, still holding a grudge against Michael and after damaging the Charger, kidnapped the burned spy to force him into doing a series of tasks in exchange for his younger brother, Nate Westen's life to obtain a biometric lock for advanced infantry weapons.
Michael was able to use details he learned on the day spent with Brennen to bluff him to spare Nate's life and leave town while enlisting Fiona, Sam and Barry's help.
The NOC List
Brennen returned to Miami after learning that Marvin "Marv" Peterson would get his hands on the list that would expose everyone responsible for getting Michael burned. He blackmailed Marv into cooperating with him. Marv's role would be to get Michael and Jesse to deliver the list under false pretenses that he could help get them both back into their agencies. After successfully extracting the list, Brennen ordered his men who were disguised as fake agents to kill Marv before disappearing.
After obtaining the list, Brennen called Michael (who invited Sam Axe) to Carlito's to discuss a new business proposition. Brennen held interrogation session audio tapes taken from Marv as leverage against Michael. The interrogation tapes contained recordings of Michael outing the people who were responsible for burning him (including Vaughn).
To prevent Brennen from releasing the recordings, Michael had to kill everyone on the NOC list. Brennen also brought in "Dead Larry" to help with the job. In the course of things, Larry successfully assassinated one of the targets with a bomb. With some misguided persuasion from Michael, Larry decided to take the list from Brennen by stabbing him in the chest, causing Brennen to die slowly and painfully with his final words being, "You didn't beat me, Michael".
Larry later intended to use Brennen's corpse as a way to unlock a biometric safe Brennen kept in Miami to store the list.
Since his debut appearance, Brennen has proven himself to be one of Michael's most ruthless and sinister enemies. He is only interested in raising money and will resort to kidnapping, blackmail or murder to help him achieve his goals. He also regularly employs a bodyguard or muscle of some kind to aid him as well. He also seems to have a long-standing grudge against Michael Westen due to the fact that Michael has managed to foil Brennen's schemes on several occasions, due to Michael's own actions and the help of his friends, Sam Axe and Fiona Glenanne. This in turn has caused Brennen to begin developing a hatred for both Sam and Fiona, who in turn have also started hating him as well due to his past actions against Michael and his family.
However, despite his ruthlessness, Brennen is very devoted to his young daughter, Annabelle who is in a private school in Switzerland and it's shown that he is fiercely protective of her to an extent that he swears revenge against Michael if any harm comes to his daughter after Michael fooled Brennen into believing that he had an assassin in Switzerland ready to kill her.