Judging from the tendency of the author to throws plot twist, Sieg probably is the shady merchant who sold the said transformation potion
I cant see a (former) lovestruck Demio in Sieg
Antboy is too chill for that
I was expecting Sieg to be secretly the demon lord, but him being demio sounds funny too... and even if he's an insect I really doubt Genzo would use him for bait, probably Carmilla will he used for that.