Playing Two Roles
Question: Can your protagonist also serve as another type of character, for example, the skeptic or guardian? Or are these types of characters supposed to be skeptical or guardian of the protagonist himself?
Keep in mind that the archetypal characters are simply conventional ways of grouping various character functions or motivations.
An archetypal Protagonist is driven to pursue and consider. The Skeptic is driven to oppose and disbelieve. The Guardian is driven to help and invoke conscience.
However, you do not have to assign these motivations in archetypal ways. For instance, you could decide to create a character who pursues, opposes, and helps and another character who considers, disbelieves, and invokes conscience. You can give a character one function or eight.
The only guidelines are...
1. You don't assign two opposing functions to the same character. For instance, you wouldn't have the same character pursue the Story Goal and at the same time try to prevent or avoid it. You wouldn't have the same character express both disbelief and faith. Putting opposing functions in different characters lets them interact and create drama.
(In the above example, the skeptic's function of "oppose" just means to speak out against a course of action - the opposite of support - whereas the antagonist's function of "avoid" means to take action to prevent the goal, the opposite of pursuit.)
2. You don't assign the same function to more than one character at a time. You don't need two characters to duplicate each other's functions.
Now, does the Guardian have to be the Guardian of the Protagonist? No. Many an Antagonist has had a Guardian or mentor figure helping his cause and keeping him on track. And other characters can have relationships with a Guardian as well.
I'm sure you can also think of examples where the Antagonist has had a Skeptic warning him that his plan is faulty (though sometimes these characters suffer for their trouble).
In fact, there are many ways you can assign these character functions in order to create dramatic interactions. The idea simply is that having all the functions present and accounted for somewhere in the story creates a very balanced cast in which no motivation or dramatic potential is left out.