Characters with a green ring on their base are Prime characters. When building a force, you may only include one Prime character on it.

What do you mean, only one? Are they like Unqiue (silver ring) characters?
They are sort of like Uniques, if that helps. But with Unqiues, you can include as many Uniques characters on your force as you like, so long as no two Uniques are the same. For example, you could include both AV 002 Iron Man and UL 089 Iron Man on the same force since they were both different characters.

With Prime characters, though, you may only have one Prime. Period. No more. If you include AS 013b Hannibal King on your force, you may not have any other Primes.

Are bystander tokens Prime? They often have a green border.
No. Bystander tokens are not Prime.