Although the Grimataur is a giant, soul-devouring monster, it doesn't stop him from going to a high school and eat food in the cafeteria. Whenever high school students sit by him, he would sap the souls out of their bodies and then trap them in the food that a human would typically eat. Then the Grimataur would devour through that human food, satisfying his hunger for souls.

... It's like putting toppings on your pizza, but in the Grimataur's case, putting the pizza on your toppings.