Hololive girls are allowed to choose their first name, but the last name is given to them by Cover. "Ceres" Fauna, "Ouro" Kronii. "Fuwawa" Abyssgard.
They usually choose their names on a whim, or based on things they like. The only odd one is Mumei. She chose the name "Nanashi," and it's not a coincidence. She thought it'd be peak comedy to share the same name as her favorite doujin artist - Nanashi/774. She was sure Cover would interfere and she'd have to change the name, but nobody noticed the connection and they allowed her to pass.
You heard it here first, believe it or not - it's your choice.