(The following character has been aged up to be 18+)

Serenity is one of the many watchmen (or in her case, watchwoman) of the Fortress of Meropide. Just one look at her and prisoners stay in line, as is the idea behind the Panopticon-like design the Meropide has.

(Notice: Here's day six of Genshin February, with an OC of mine. She's based on concept art for a Fontaine character many people thought was Chevreuse. Here's the link: https://preview.redd.it/concept-art-references-v0-xh7n8xy18uhc1.jpg?width=858&format=pjpg&auto=webp&s=688c3d07c9eb1e0c57291e2eb01b5f894b1c5bc5.)