There are humans on our side; mostly those who have been betrayed by their fellow humans or disappointed by the hypocrisy of humanity. Most of them are powerful people too, so they are highly wanted betrayers.
Because of that fact, seeing humans among us is not weird. Although the only human on our side that I have seen so far is only Millonia and, possibly, Valeria; I have heard stories that they get along well with the Monsters and Demons.
My troop mates have attested to that claim, so I am very sure other than becoming a hostage, we don't have human slaves. See, even at the castle, Ilschevar employs Dark Spirits, who can do a way better job than humans.
In conclusion, I don't understand what worth Velucan has seen in the two women that I failed to see. Both of them are far from being strong; on top of that, they also don't look like they are the type who can be bossed around, so they will make a bad worker.
"What are you going to do with them?"