Westley might be considered an unusual character.  While his mythology and background could be seen as a typical hero, Westley really is not the normal hero.  He endures insults from Buttercup and starts off in the lowest rung of the social order.  While he experiences the "derring- do" of swashbuckling ends, he does so rather inadvertently.  It's out of being kidnapped that he ends up living the life of the Dread Pirate.  Westley is not conscious of being a hero or a heroic figure.  This is what makes him unusual.  We look to him as a figure or leading protagonist, but he is not really one who sees himself in such a manner.  Part of why he is like this is that Westley never fully embraces his love for Buttercup.  This makes him unusual in the narrative.  Traditionally, the reader is accustomed to a character who is able to embrace their "destiny" to love the damsel.  Westley is unusual because of his divergence from this configuration.