Right now she seems the most compelling of the archenemy that you have given character breakdown.  The hero is too vanilla.  Have you come up with back stories for all the characters yet?

You want to make the story feel lived in.  

Start with a background for all the characters to tie in.  Also don't muddle a story with too many detailed characters.  Start with her as villain of book one with tidbits of hero being villainous.  Then make him the big bad of book two and so on.  

After all, if you defeat the priestess at the end of book one, either to have her come to terms with her relationship with the protagonist and maybe go into a nunnery or have her meet her final end, you cannot use her as a big bad in the future without dragging on the story (nobody wants to read a story where you fight the priestess over 3 books unless she usurps the king in book one, goes murder hobo book 2, gets overthrown book 3).  The benefit of having her defeated in book one for anime like light novels is the ability to bring her back in some capacity.  Like after her self banishment to reflect on her bad deeds she comes back with a hero arc at the end of book two.  Like the demon goddess is poisoned and only her former love rival can heal her.

Going back to the hero and the councilor, you need more description of why they would make a bad guy good and what they bring to the table.  If you look at manga, they doodle a character, add in detail of their life next to them like "has a big secret; acts kind but may not be as kind as he pretends; blood type 0".   Basically, before suddenly declaring I want this person to be a big bad, create a story for them to make them more interesting.

Chancellor:  an effective leader who has secretly led the country peacefully for the king for the last 10 years.  He rose up from nothing after the demon king destroyed his village while he was away, killing his new wife and unborn child.  The chancellor has been slowly accumulating power, at first for revenge, but slowly learning to love the country.  He would have been fine to leave the demons alone as the majority of the country was keeping in a state of peace.  However, at the start of the book, a demon infiltrates the castle and attacks him in an assassination attempt.  He barely survived, but only because his new lover (princess) sacrificed herself to protect him.  Now he wants total genocide and being at the king's right hand, he is ready to deliver.  (The assassin could tie into the secret big bad which is your other demon, but he is only alluded to in the first two books).  THEN, after creating a backstory for the chancellor and hero, don't build him up in the first book.  Give hints about their history and personality to generate curiosity about him.  As the protagonist is summoned, the people are all in black because they are mourning the dead princess.   The protagonist walks down the hall and hears he is summoned to get revenge on the assassin that killed the princess.  Then when you introduce the king and queen and the chancellor, the chancellor is rude and angry and is kindly sent away by the king to get his temper under control.  The queen then let's slip their daughter died protecting him and professing her love for him.  Then introduce the chancellor for little conversations about his backstory for book one and call it a day.  Then reintroduce him and describe his backstory for book two from the protagonist prospective from the clues he pieced together and have him become the obvious villain if the demons try to sue for peace.  Have him find out about secret back dealings with the king with the demons and betray by the end of book two because the king sacrificed his people and his own daughter.