There are two types of special characters: Unique and Prime.
If a character is Unique, only one copy of that character can be part of your force. Unique characters have a silver ring on their bases (older Unique characters may have a gold or bronze ring) and a silver background in the character window on the character card. A force can’t contain two Unique characters with the same name, the same set symbol, and the same collector’s number.
Characters with a green ring on their base and a green background in the character window on the character card are Prime characters. When building a force, you may only include one Prime character on it.