Rosetta is my second favorite character after Sora, so definitely her. Marion is cute and I like her too though.
I wish they had shown their show together as new partners in the extra episode, especially because she was fated to be with Sora and do the legendary manuever again :)
This will be a landslide for the shows #1 tsundere, Rosetta Passel. I liked all episodes focusing on her and how she began to look up to Sora as her big sister or something. Started out as a young prodigy with quite the attidute, became more open and was revealed to have quite the childish side to her after. I also liked how she decided to develop her act as well, going from diabolo to trapeze. Overall, some good development concerning both her character as well as talents.
Marion isn't actually that bad or anything. It's just that Rosetta was given more time. She has tie ins to Kaleido Stage since birth and is basically living there and thus is also present in lots of scenes. However, there isn't that many episodes devoted to her (maybe just one actually). The best one would be the one dealing with her father and her mothers demise. She also befriends the shows mascot animal and can apparently talk seal, which is somewhat amusing. She also develops some acting on the kids stage, so I guess I'm satisfied overall even though she is one of the less important characters.