Who is the chief of the metalbending police in Republic City?

Lin Beifong is the chief of Republic City's metalbending police force. She is one of the city's toughest law-enforcement officials and uses both earthbending and metalbending in her work.

Lin inherited the role's legacy from her mother, Toph Beifong, who pioneered metalbending and previously served as Republic City's police chief. Lin wears metal cables as part of her equipment, allowing her to swing through the city, restrain suspects, and fight at a distance. Her policing style is severe and uncompromising, but she is also deeply loyal to the city and willing to sacrifice herself to protect civilians.

Toph is Lin's mother, not the answer to the question, because Toph belongs primarily to the earlier Avatar: The Last Airbender era. Suyin Beifong is Lin's younger half-sister, while Kuvira is a later military and political leader associated with the Earth Empire. Lin's complicated family history becomes a major part of the story in later books.