Curtis Manning (Seasons 4-6) 3

Curtis is one of the main CTU agents during his time on the show, but his role is mostly as an assistant to Bauer or whoever is leading CTU. He isn't directly involved in many of the storylines, but most notably its his history with a terrorist which gets him shot by Jack Bauer in season 6. Manning is assumed to be dead, but its never officially revealed on the show.