A Rebel is Mint Green.
The Rebels first apperance was in Part 2, where two of them came to the Dardrain Desert to help Blue fight the king. Unfortunly, they were quickly killed by Sheriff James.
Later on, in Part 4, they helped Blue attack Black's castle. However, when Blue discoved that Black had survived, and returned, some of them went to fight the king.
In Part 7, they joined Black's side. The reason is unclear.