What kind of demon is in "My Status as an Assassin Obviously Exceeds the Hero's"?
As noted in @AkiTanaka's comment, the word translated as &quot;Demon&quot; in this manga is &quot;ma&quot; (魔), or &quot;maō&quot; (魔王) for Demon King and &quot;mazoku&quot; (魔族) when discussing the demon kingdom.
I haven't personally read the Manga, but you can tell by comparing these panels from the first chapter:




So no, the &quot;demons&quot; to which they're referring aren't a particular type of being from Japanese mythology, like &quot;youkai&quot; or &quot;oni&quot;, but rather a more general term for evil spirits/beings often used in generic Fantasy settings.
It's the same &quot;ma&quot; used in &quot;akuma&quot; (悪魔), which you may have also seen in many Manga and Anime, and is often aligned with more Western/foreign notions of &quot;demons&quot; and &quot;devils&quot;.
See e.g. this answer for some nuances of how of these various terms differ.