Take what I say here with a grain of salt, since I read it a while ago from someone else's spoilers.
Iirc from another comment, Ian was an illegitimate son from a northern lord and southern woman. Because he had no standing, Michel's father used Ian as a sex slave from a young age. Later, Ian's northern family found him. They didn't treat him any better, but he rose to power.

why the duke ian go this far by pretending to be ain as a servant though? do he worried if michael intimidated by him if he says he is the duke from the start? the arts are amazing but really wish to know the reason ʕᵔᴥᵔʔ