No specific gender or look is necessary for either, but a couple notes to bear in mind: Astaroth is inherently a deceitful type, and while he rarely (if ever) lies outright, he does do a lot of lying by omission, speaks in manipulative ways, and employs plenty of flattery and appeals to superiority to get what he wants. As the "prince of accusers and inquisitors," Astaroth favors those who are bold, intelligent, decisive, and not easily swayed in their opinions or belief. Other traits that might be useful for his minders to have (or would at least make their lives just a little easier in dealing with him) include resistance to his bribery and seduction, as well as a willingness to put up with his laziness, vanity, and general tendency to slither out of things.

Also! Astaroth's master minder will gain the power of invisibility, while his apprentice will gain dominion over serpents (including the ability to control and communicate with them).