To quote Wikipedia:
The show had an unusually high turnover rate among the regular cast, partially due to contractual disagreements between the cast and the producers.[
citation needed] Almost all of the show's major characters were killed or otherwise removed within a season or two of being introduced. In fact, the only character to appear as a regular during all five seasons was FBI Agent Ronald Sandoval (
Von Flores), one of the show's antagonists.
S5 was a major overhaul from the previous 4. The character "Liam" replaced Boone in S2 and lasted until S4 but unfortunately they made him more and more dull as the show went on. (If you even found him that interesting, he... wasn't always the best actor.)