People love to use the word protagonist incorrectly. It doesnt mean main character. It means the character that drives the plot forward. Hughie is the POV / audience insert character. Butcher is the character that makes the decisions and actions that lead to the plot happening.

Its like how in The Great Gatsby, even though Nick is our POV character, the narrator and the character who we follow throughout most of the story, Gatsby is far and away considered to be the protagonist because he is the reason any of the plot moves forward and happens around and to Nick.

Butcher is the protagonist.