Can a Main Character also be the Sidekick of the Protagonist?
Question: Since in the story that I'm thinking separated the Protagonist and the Main Character, is it possible to give my main character the role of the sidekick? Since he (the Main Character) deeply believes in the actions of the Protagonist. And also, Can I also give my Protagonist a role similar to that of a Guardian? (There are going to be multiple Guardians the Main Character is going to get acquainted to). My idea is to have the Protagonist act like a Guardian in the beginning of the story develop the Main Character since the Main Character is the key to achieving the Protagonist's goal in the end. So is this okay?
Absolutely the main character can be a sidekick, as long as his main function in the story is to be supportive of and show faith in others (such as the protagonist).
It is also possible for one character to have the traits of both Protagonist and Guardian.
However, what you have to consider is whether your main character isn't actually evolving into the Protagonist (even if it doesn't seem so at this stage).
For example, in Star Wars
Obi wan Kenobi seems to take over the lead role in the effort to deliver the Death Star plans (after R2D2), while simultaneously acting as Guardian to Luke. However, Luke ultimately takes on the role of Protagonist and destroys the Death Star at the climax of the story.
Sometimes, these roles are only clear once the entire story is finished and one can look at the whole (and possibly do some revision).
There is a natural tendency as well for the main character to take on the Protagonist role, since presumably the main character's decision of whether or not to change has a big effect on the outcome.
If you want to keep the main character as the Sidekick and not the Protagonist, then you will need to be careful to keep him in a purely supportive role--but you may find at some point that you don't want to.