Rosaria is one of the new characters players can meet through the Dragonspine content added via The Chalk Prince and the Dragon, the Ver 1.2 update in Genshin Impact (PC, PS4, PS5, Mobile, Switch soon). Screenshots showing Rosaria were first officially published during the ver 1.2 stream.Unlike Albedo, Rosaria isn't playable. However, judging from how a game like Genshin Impact works, she will definitely end up featured on a gacha banner at some point. It's also officially undisclosed if she is a four star character or an harder to obtain five star character.

The Genshin Impact Japanese official Twitter introduced Rosaria once again now that ver 1.2 launched:

The tweet explains Rosaria is a sister of the Favonius Church in Mondstadt. However, she definitely doesn't act like one. She never prays either.

The tweet also revealed the seiyuu voicing Rosaria: It's Ai Kakuma, known for voicing Edelgard in Fire Emblem Three Houses. Ai Kakuma also voiced Ion in Surge Concerto, Isuzu Sento in Amagi Brilliant Park, Sora Ikushima in Tokyo Xanadu, and Rami Amasaki in Super Robot Wars T. Among other roles, she's also the current voice of Yuri Sakazaki in SNK games, since KOF XIV.

Players can meet Rosaria for the first time soon after starting the new story content added with Dragonspine. The traveler and Paimon eavesdrops on her conversation with Barbara, and late properly speak to each other for the first time.

Lastly, you can learn a bit more about Rosaria and many other unrevealed characters in-game. By inreasing Friendship levels. For example, if you do so and ask Barbara about Rosaria, she mentions she's sure Rosaria believes in Barbatos even if she doesn't pray.

Following Albedo, the Ganyu banner will start in mid-January. miHoYo also detailed the changes Zhongli and Geo characters like Albedo will be affected with in the Ver 1.3 CBT. However, everything is still subject to change until February 2021.