I think that Berkut is a fairly well-written narcissist who spends the entire story being shown that there's someone better than him in every way that he considers important.
Berkut's writing is not bad; but he dies before it has a chance to really resolve.
I'd like to refer to the five stages of grief in the context of Berkut: denial, anger, bargaining, depression, and acceptance.
Before the climax of Berkut's character arc in Act 5, he lost to Alm twice in Acts 3 and 4. This showed him that Alm was a better fighter/general than him, something he thought he was best at. I posit that he could justify "Oh well, I'm still royalty and he's just a commoner- i'm still better than him." He denies. Even though he lost to a commoner, he's still royalty. But then - surprise! - Alm is the prince of Rigel. Now HE'S below him in both station and competence, his two most treasured attributes. He throws a complete tantrum, lashing out in anger towards Rinea. Finally, in act 5 he bargains with Duma for the power to destroy Alm (and possibly the country that scorned him for Alm). Then he dies upon defeat at Alm's hands, with a shoehorned acceptance as Rinea guides him to the afterlife. Even though i think that's pretty cute.
I think Berkut could have been resolved better by being playable, which I've heard was once a thing. Then he could made his bargain earlier, had a bout of depression afterwards (not about losing this time, but about sacrificing Rinea), and accepted his shortcomings later and joining the cause to avenge his lover. Plus, we may have seen Rudolf react to his demonic powers, which I think would have given his true motives some much needed foreshadowing (even if it's not so obvious as to give away the twist).
Fernand isn't terrible, but he's still pretty lame.