You never see the protagonist in any VN. Besides, they did a good job of describing it, or at least her "Chimera-ism". She looks like Shiki with a trenchcoat first because she forcely and unknowingly pulled the piece of Chaos Arc used to heal him up, then she starts looking like herself, later on the markings of Nero (twitching facial veins, the blackness appearing as a cancer below the neck) appear on her.
What she actually looked like before she was eaten though... well, just think of her like uh, generic-looking girl. Like Tenma.