Willpower
If there’s one thing you should understand about the Kurōn its that their will is significantly weak to those who are stronger and powerful. They were created to serve and o b e y, and act on the will of Subete no hahaoya. But there is a flaw in their design, Hahaoya did not anticipate humans ( or other creatures concerning AU’s ) having a strong enough will to usurp her hold on the Kurōn. So if a woman or man comes along with a significantly stronger will than the Kurōn in question ( like Nana ), she will immediately o b e y.
Willpower isn’t immediately sensed, however, for the Kurōn – it’s more like an assertion of dominance. A command given to the Kurōn for which they cannot disobey and immediately follow the order; its an intoxicating feeling for the alien species in particular, and they feel rewarded, happy and full of p u r p o s e when told what to do.
This also feeds their more sinful desires; sex, gluttony, obsession, etc. They have no concept of what is ‘too much’, only what they think is 'too little’, and because of that many of them take on a characteristic of something they feel isn’t enough. In Nana’s case; it’s food of any kind – doesn’t matter if its sweet, bitter, tangy, tasteless, she has an insatiable need to eat and fill herself up.
Ironically, food other than the flesh of humans can truly satisfy her. So it’s an endless cycle of stuffing her face and feeling completely empty.