...character?
It doesn't have to be, by the way, a "good guy" from CTU or anyone who helps Mr. Bauer. It could be a badass villain, or even one of those people who are neither heroes or villains but just men or women who got caught up in the events of a particular season (or Day).
My favorite? Hard to say, since they're almost all pretty interesting. I will put up my Top Five, to get people thinking and maybe get something going on here.
1. Chloe O'Brien
2. Tony Almeida
3. David Palmer
4. President Charles Logan
5. Agent Aaron Pierce