what is the role of sergeant in bishop's candlesticks?

Yours is a very valid and nice query. Shows that a detailed study has been done on the chapter. Although the sergeant played a minor role in the play, his importance cannot be ignored. His introduction in the play is indirectly related to the change that came up in the life of the convict. If it wouldn't have been for the Sergeant, the convict would have been convicted and imprisoned for his crime. It was because of the Sergeant bringing him back to the Bishop that the convict was given another chance in life, when the Bishop distorted the truth and introduced the convict as his friend. The Sergeant is shown to be of a very good character because even after arguing a lot upon freeing the criminal, he actually passed on his custody to the Bishop. That is the reason which helped the bishop give a chance to the criminal to amend his life.