I played a lot of Marth during my most recent Smash sessions, alongside my Rosalina, and he's the character that's most similar to Rosalina to me, due to the fact he has a heavy focus on spacing and requires precision and attention to detail to play well. But having a good ability to space really pays off with him, much like with Rosalina. His frame data also feels fairly similar (with long aerial animations, for example), though he does feel a bit stiffer and like you're more susceptible to being punished after a whiffed aerial and what not. That's partly due to the fact he also falls faster. His walk/dash speed is also fairly close to Rosalina's, so that doesn't really require much adjustment either.
My Rosalina isn't 100% defensive though, in fact how I play is just dictated by the pace of the match, and I normally default to an offensive/defensive balance, and I try to mix people up. So that's partly why I feel Marth is most similar for me. If you play a mostly defensive/campy Rosalina, then other characters might feel closer to how you play her, such as Villager.