Greg Pak 
Firefly: Blue Sun Rising #0 [PDF ebook] 

Sheriff Mal Reynolds has a new partner-a law enforcing robot from the Blue Sun corporation, who doesn’t care about motives, about mercy, about anything other than enforcing the law-no matter the cost. The Blue Sun Corporation has helped to run the ‘Verse from the shadows for years, but they’re ready to step into the light and take over. If Mal wants to keep his job and protect his sector, he probably should play by their rulebook. That leaves only one choice for Mal -reunite the crew of the Serenity for one last impossible job to save the ‘Verse.