Edit: This post was made before the release of the third book in the Broken Code series. It was later proven to be right.
Don’t read this if you haven’t read the third book in the Broken Code series!
I think I know who the imposter is...
I think the imposter is Ashfur! I know that is strange, but think about it, Shadowsight noticed that the imposter’s eyes are blue. In the books, Ashfur’s eyes are blue. Ashfur had a crush on Squirrelflight, so it would make sense that he would want to be her mate. He was also mad at her because of Lionblaze and Jayfeather, because she lied to everyone and said they were her kits. Crowfeather was also included as a codebreaker because he had kits with Leafpool. Do you agree? Who do you think the imposter is?