Disclaimer: The information below is mostly speculative and uses data from unreleased characters obtained from honeyhunter and other leaks. These are all subject to change when it's released.
Rosaria is a Cryo polearm damage dealer that deals a mix of physical and Cryo. This can mean that she can be a strong physical carry because it's easier to cause "Superconduct" reaction with just 1 Electro teammate, or a strong Cryo burst from her skills by bringing a Pyro teammate. Compared to the other 4 star characters, her damage numbers seems to be about average, but the cooldown on her elemental skill is fairly short, and her elemental burst has potential to deal significant damage. But combined with being able to use both Superconduct and Melt gives her some really good potential.
Fischl: One finds Sister Rosaria to be a most devout believer in the Anemo Archon. Oz: ...""Devout believer,"" mein Fräulein? Fischl: Indeed! One night, I heard her invoking the name of the Archon thus in another's presence: ""May you be struck down in the name of Lord Bartabos!"" One presumed that she was leading a prayer.
Some may see Sister Rosaria as one who complains about how bothersome her work is, but she does her job well. Last time I saw her I told her she could come find me should she ever need help, to which she simply replied that doing so would be even more bothersome. I need to work harder to become of more assistance to the people of Mondstadt.
Ahaha, you want to know about the secrets of Sister Rosaria? That... I cannot tell you. Though my impression is not bad — certainly better than a certain someone whose name I won't bother to mention. She's honest with herself. She casts off that which she has no time for and she doesn't feign motivation for affairs that do not interest her.
Sister Rosaria is hard to get to know, especially given she never joins in on prayers, but... Oh, I'm sure she believes in the Anemo Archon! Although, I still don't really have the courage to try to talk to her...