Spoiler :
the real villain is Jermel. He wants to collect all the sigma that exist
The 2nd prince, he's just one of jermel's pawn. The 2nd prince and Callipa (MC's friend with black dress) they used to be engaged.. the reason why the engagement was broken off was because Callipa's family assassination.
It turns out the reason of her family's death was Jermel.. i don't exactly remember why he killed her family but thanks to that, Callipa gained a sigma and now, Jermel want to have her sigma as well
About the relationship of 2nd prince and Calipa, he's actually in love with her but she doesn't have the same feelings towards him.. when he found out that Jermel killed her family, he feels regret.. he didn't know that Jermel would act on his own.
Jermel and 2nd prince has a plan to revolt using the black army.. when callipa is in the black army, that means if the plan succeed, then 2nd prince can be the emperor and Callipa as the empress
But since the princess wants callipa to join her army, that means they can't be together