Ghoul

Ghouls are only able to eat people. Any attempt to eat others foods will make them sick because of an particular enzyme that they have. The structure of their tongues are also different from a human's, making other foods taste disgusting and uncomfortable.

Ghouls can pretend to eat food in order to fool other people into believing they are human. They have to make themselves throw up soon afterwards, or they'll become sick.

Ghouls don't need to eat in the short term like humans do either. They can survive for a good month or two just eating one body. However, some ghouls eat more simply for pleasure.

While they can't eat normal food, they can drink coffee for some unknown reason. Through various scenes in the story, it can also be seen that they can drink plain water.