Boss Moon was a character who appeared in the pre-Serenity (film) arc of the Boom comics. She was one of the Unificators tasked with hunting down Malcolm Reynolds. She was a cyborg, part human, and part machine. She and Reynolds ended up calling a truce long enough to get through the situation they were both trying to survive while surrounded by renegade Browncoats and Alliance soldiers.
Moon ended up partnered with Reynolds when he was made sheriff in the area. The Alliance and Blue Sun Corporation were worried about a string of robberies on Blue Sun trucks in the area and wanted the activity stopped.
Moon eventually defected from the Alliance, aiding the raid on a Blue Sun factory against the patrol bots and offering some of her cybernetics to keep Serenity flying. This endangered her life, but she survived and settled on Haven. This may well mean she died in the attack on the settlement, but it’s not confirmed.