Ivanova (the Cynic); Sheridan (the Optimist); Garibaldi (the Realist); Franklin (the Conflicted), and Marcus (the Apathetic).
Alternatively, Franklin could be seen as a second Optimist, since he has some of the stubborn and optimistic traits, but is not as determined or decisive as Sheridan. Ivanova and and Garibaldi also sometimes share both the Cynic and Realist roles.
Ivanova insists that she's a Realist, but even she admits that's because she's Russian.
Optimist: David Lister, always hopeful in the crew's plans, almost to the point of blind hope in the absence of no logical certainty that the plan will run smoothly. Will be the first one to support looting a spaceship on the verge of exploding, just so he could get a supply of curry.
Cynic: Arnold Rimmer. He'll pretty much say no to any plan, percieving it to be too risky. Has practically no faith in anything.
Realist: Probably Kryten. He'll acknowledge that Lister is being overly optimistic, and that his plan probably is suicidal, but yet informs Rimmer that even though a mission might be dangerous, they are in danger (of possible low food supplies) and must take the chance to survive anyway.
Apathetic: The Cat. Although he does get better, he's fairly indifferent to most of the others' discussions, and lives in his own mental world.
Conflicted: This role appeares to be taken up both by Holly in early episodes, and then Kochanski later. It ranges from high hopes in plans, that could end up leading the crew into hazard, or outright saying many plans won't work. In a way, this could be the cause of the character's instabilities.
Vengeance: Spartacus is The realist, Crixus is The Optimist, Agron is The Cynic, Oenomaus is The Conflicted and Gannicus is The Apathetic.
War of the Damned: Spartacus has become The Optimist, Crixus became The Cynic, Gannicus is still The Apathetic and Agron becomes The Realist.
Star Trek: Voyager: This probably applies to the rest of Star Trek shows. While there are obviously more than four or five characters, the overlapped roles have characters that fall under different aspects of each role. For example, Tuvok is the coldly-logical Cynic, Paris is the street smart Cynic and Torres embodies raw hostility.