Like a good warrior, Caspian needs his guide or mentor. Unlike Luke Skywalker, Caspian gets more than one Yoda to fill the position.
Dr. Cornelius serves as the mentor of knowledge and wisdom since he teaches Caspian about "the old days" (5.1). He later pops up to advise Caspian as part of his war council. Peter acts as a guide for Caspian on how to be a proper and chivalrous king. To do this, Peter fights for Caspian's cause but also serves as a chivalrous role model for the to-be king.