This is a tricky question. I could reduce it to "Elliot usually tops" but their power dynamic is a bit more convoluted than that.
Elliot usually tops because Leo wants him to, not because he's "the top".
You could say Elliot is a service top with a big praise kink.
Leo is lazy and lets Elliot do all the work usually, he's a bit of a pillow prince, but he's the one who actually "directs the play".
They don't reverse their usual roles in bed, Leo is still the Lord and Elliot still the obedient knight who'd do (almost) anything to please his lord (and be praised).
I think in the fic you'll see them switch a few times anddd quite a bit of fellatio? Because I like it and Elliot is very good at it eheh (and Leo really likes seeing him on his knees).

This is a very nice question, thank you! For favourite I'd say probably Lottie. She doesn't get a lot of screen time but I really love her as a character and I like to imagine her little shenanigans at the castle. I'm also very fond of a character we made up: Noel. He is one of Vincent's cadets and I kiss his forehead everyday (I'd like to say more but he'll be a plot point later on, so just know he's a good boy). Oswald, Lacie and Lottie interacting are the funniest to write lol. For single character I'd say probably Leo. Elliot is the easiest alongside Gilbert! (You didn't ask but the most difficult to write are Levi and Jack afhofegfe)