Secret characters are special opponents who are usually not found within the original 1P and 2P Battle Plan. To reach it, one must meet specific conditions, often challenging ones in order to face them. For the most part, secret characters are unplayable and merely exist to challenge the player.
However, in Rise 2: Resurrection and the director's cut, secret characters can be unlocked for player control via the Code.
List of Secret Robots
Rise 2: Resurrection