Super Smash Bros. Brawl features a single-player mode known as The Subspace Emissary. This mode features unique character storylines along with numerous side-scrolling levels and multiple bosses to fight, as well as CG cut scenes explaining the storyline. The Subspace Emissary features a new group of antagonists called the Subspace Army, who are led by the Ancient Minister. Some of these enemy characters appeared in previous Nintendo video games, such as Petey Piranha from the Super Mario series and a squadron of R. O. B. s based on classic Nintendo hardware. The Subspace Emissary also boasts a number of original enemies, such as the Roader, a robotic unicycle; the Bytan, a one-eyed ball-like creature which can replicate itself if left alone; and the Primid, enemies that come in many variations. Though primarily a single-player mode, The Subspace Emissary allows for cooperative multiplayer. There are five difficulty levels for each stage, and there is a method of increasing characters' powers during the game. This is done by placing collected stickers onto the bottom of a character's trophy between stages to improve various aspects of a fighter.