Anti-Prime is a term used to refer to a character that exists in a parallel Universe to that of the Primary (Prime) Universe. Anti-Prime characters and things in general are usually the polar opposite than that of the original, and are sometimes role-reversed, meaning they also no longer carry the same occupation or role as the original character held. Anti-Prime characters are not usually involved until someone figures out how to jump dimensions.
The idea comes from a classic plot device used in Western comic books and series, where Anti-characters serve as a manipulative villain figure to the original heroic character. Typically, heroes are thrown into the reverse world or vice versa and other characters confuse them with one another. The arc ends with both alternates meeting and colliding, often sending the original back to where they came from.