You can unlock Student Predators (All except Ligeska the Snake woman), and for Bella, the only way to unlock her as a playable predator is to beat her stage with the book in hand.
However, playing as any of the unlockable Student Predators does not mean that you’ll become an Apex Predator; you’re still susceptible to becoming devoured by the apex regardless.
If you have a full belly (As a Student Predator), you can rub your own belly to help digest just a little faster.