In the manual, it's stated Sonia "learned at an early age of her ability to sense the presence of beings physical and spiritual that cannot be seen by ordinary humans."
At first, I thought this was something that wasn't used in the game itself. However, what if all of the enemies themselves are the beings "that cannot be seen by ordinary humans"? That might be the primary reason that the Belmonts go up against Dracula: no one else can even see these monsters. Other magic users such as Sypha, Carrie, Charlotte, and Shanoa could be assumed to have the same ability. Alucard can, because he's half-vampire. Eric, because, um, the Alucard Spear? But what about regular guys like Grant and Nathan?
What do you think?