The three mercenaries hired by the Tosatsu Kingdom are demons, and are considered to be evil, violent creatures, according to Fukami. Many characters created by Mogeko are demons; the majority of them have horns, and many of them have bat-like wings.
Demons are prevalent in Mogeko's game The Gray Garden, and comic series Obsolete Dream. According to The Gray Garden, there are multiple dimensions in which a demon and God rule, and demons and angels coexist in different ways. Chlomaki appears in Obsolete Dream and knows the main character, so it is possible Wadanohara and the Great Blue Sea and Obsolete Dream take place in the same universe.
Demons and angels apparently don't get along in many of these dimensions, and the dimension featured in The Gray Garden is a rare exception where they coexist in peace.