The FC Demon has dark pink skin, black eyes, lilac hair, long pink horns and lilac/black clothing that includes a checkered top.
They can be encountered in FC World B. There is a 1-in-6 chance that the FC Demon will appear in the place which has its entrance surrounded by six pyramids. However, they will always appear if Madotsuki has the Demon effect equipped. If interacted with at the pyramids, the FC Demon teleports Madotsuki to their room in the FC Basement, to their side.
When interacting with Madotsuki, the FC Demon makes a sound that sounds similar to a smooch. They will always turn to look at Madotsuki when interacted with.
- While the FC Demon themself does not resemble the Japanese folklore creature Oni, the Demon effect they give out transforms Madotsuki into strongly resembling said creature.
- In the FC House, an adventurous tune is playing, but the background music in the FC Demon's room is calmer and more cheerful, and features sounds that are akin to birds chirping. In the official CD, the described track is named "The Bird is outside the house and the Demon is inside the house".