The Mattress Demon is an evil spirit that resides with a mattress. She makes her presence known by manifesting a face to frighten Courage. She later takes possession of Muriel, changing her appearance to green skin with wavy orange hair and a deep, demonic voice. Courage and Eustace spend most of the episode trying to get rid of the evil spirit. Courage uses the incantation to drive the spirit out of Muriel, only for her to possess Eustace minutes later. Muriel knocks him out and orders the mattress to be taken back.
- It is unknown what the demon was doing to Muriel, but judging by her shout of "Courage, HELP ME!" when she suddenly reappears, it must be something bad.
- Despite the demon assumably being female, she is voiced by Jim Cummings, a male.