Brennan was first mentioned in "Pilot, Part 2", when the survivors listened to Danielle Rousseau's transmission. She mentioned that she was in the radio tower and that "Brennan took the keys." He finally appeared for the first time when he, Montand, Robert, Nadine, Lacombe, and Danielle discovered a time-traveling Jin stranded in the ocean. They rescued him and brought him to shore. ("The Little Prince") While traveling to the radio tower, Brennan and the rest of the team were attacked by the monster, who killed Nadine and abducted Montand after ripping off his arm. Brennan, Robert, and Lacombe followed the monster into the hole. Some time afterwards, they presumably began to act differently due to the sickness, and they were killed by Rousseau. Brennan's corpse, along with Lacombe's, was found on the beach by Jin, with three bullet wounds in the chest. ("This Place Is Death")
In the official ABC press release for "The Little Prince", Brennan was credited as Frenchman #3.
"Brennan" is also the last name of Kate's childhood sweetheart, Tom.
It is interesting to note that the actor who portrays Brennan, Bruno Bruni, shares the first two letters of both his forename and surname with the character he portrays.
What are the keys Danielle mentions in the transmission?