Richard Ryder is actually Nova Centurion Prime, and he's not even that in this movie, since it's still Rhomann Dey at the moment.
Glenn Close is Nova Prime. I'm guessing she's like the equivalent of the "Sergeant Major of the Marine Corp" whereas Rhomann Dey would be like the "Sergeant Major", who would be the top enlisted commanding officer of the marines. "Sergeant Major of the Marine Corp" is a top advisory/leadership position that isn't commissioned, I think. I could be wrong though.
Where it gets confusing, both with the Marines and the Nova Corp is that the "Sergeant Major of the Marine Corp" is often abbreviated to just "Sergeant Major", and likewise, "Nova Centurion Prime" gets abbreviated to "Nova Prime".
And of course, Richard Ryder just gets abbreviated to "Nova" as if that is the character's name.