Nightmare

You can meet him in the forest's "Grey Flower Field". Hidden behind a mask, you can't see his bare face, but his eyes reflect a certain sadness. Transparent, it's not possible to touch his body. Only Tiana and the Seven Dwarves are able to see Nightmare. Much like the other residents of the cursed Lost Forest, Nightmare's true name is a mystery, and was placed by the dwarves.

Spoiler

Nightmare is actually the (human) prince of the country the story takes place in. According to his ending bonus "Secret Memo", Nightmare's real name is Carlfriedrich and he is 18 years old. His favourite food is mint tea, favourite colour is light blue and his hobby is horse-riding.