8. Jimmy Borelli
This one is going to ruffle a few feathers. Jimmy Borelli (Steven R. McQueen) was not posed as a villain on Chicago Fire, and he had a much longer run on the show than the previous two entries. The thing is, the longer he stuck around, the more he proved to be a thorn in the side of the firehouse.
He constantly butted heads with Chief Boden (Eamonn Walker), culminating with the incident in which he ignored Chief Boden's commands and suffered a career-ending injury as a result. Then there was the time he blamed Boden for the death of his brother.
Borelli wasn't a bad guy per say, but he was a character who suffered greatly from writing, and wound up be pretty unlikeable as a result.