Progress Fifth is the hardest boss in the game. He is currently the only boss in the game who resembles a human, Sir GreenGrass excluded due to the fact of how he wears armor. He is the fourth and final boss you fight in the game,
Progress fifth counters everything, making it a really annoying boss to fight. He attacks with a melee and ranged weapon, and therefore can counter ranged cubes. He also doesn't die to the void, and this mechanic was added because of how he sometimes backflips.