Shirahoshi, from One Piece. But she's a child.

King Neptune hired You.

Shirahoshi is a big girl, due to a growth spirit. She's now she's a big mermaid child. In the future, she'll grow even bigger than all the mermaids, besides her father.

Shirahoshi locked in a room, with weapons stabbed on the door and walls in front because of Vander Decken IX Devil Fruit, which can target the person, and he can throw weapons at her from across the word. She's locked in the room to protect her from him.