A red herring is anything or anyone that is intended by the writer to be misleading and distracting. As a character, a red herring has the potential to complicate an otherwise simple plot. They can be mistaken for the antagonist or any other characters whose identities are unknown at the start of the story.
If you want to have a character be mistaken for a different character in a specific role such as the antagonist you need to make sure that they are believable in their mistaken role. Fake antagonist? Make the red herring arrogant, cruel, or sadistic. If they are a fake murderer make it so that they are an average kind character who was just in the wrong place at the wrong time.
Another aspect of writing a red herring character is how being mistaken for someone else can psychologically, socially, and financially effect them. If you were mistaken for a murderer how would you feel? Especially if you got put in jail until the mix up got cleared up. It is important to think about how the role of any character in a story can negatively affect them.