The first several "Devil" robots were created by Dr. Wily to serve as some of the last lines of defense for his castles against Mega Man's advance. In future time periods, however, more advanced models have shown up, beginning with the Shadow Devil in Mega Man X5. No matter the name, shape or color, they all share one trait in common: A singular red eye situated within a malleable body made out of an unknown substance, the eye itself generally being its weak point, as the rest of the body is usually impervious to all attacks. Despite their apparent shapeshifting abilities, most have not shown a wide range of forms when in battle. Their main attack has often consisted of breaking themselves up into pieces that dart over from one side of the room to the other and then reforming in that spot. Other than their color, however, what distinguishes "Devil" robots from each other is their sturdiness and the fashion in which they attack.
Most of the "Devil" series robots have the distinction of showing weakness reactions towards Electric-type weapons. The Green Devil and Shadow Devil bosses from Mega Man 8 and Mega Man X5 also have an electric-based weakness: Thunder Claw and Tri-Thunder respectively.
Block Man's giant form appears to be based on the Yellow Devil, having a similar pose from its appearance in Mega Man: The Power Battle, a red core as its weak point, and saying "bumo" after transforming in the Japanese version.
List of Devils
In the first Mega Man game, the Yellow Devil's only attacks were shooting its mass as a series of projectiles that would dart from one side of the room over to the other in a set pattern, as well as a small projectile fired from its eye while whole. It was weak against Elec Man's Thunder Beam.
In Mega Man Powered Up, the Yellow Devil broke up into a whirling torrent of spheres before flying over to the other side, although the battle was fundamentally the same, save for an eye laser attack, and the hand crush attack when the player stands too close to it. This time, it is weak to Fire Man's Fire Storm.
Yellow Devil MK-II
Despite its inability to defeat Mega Man, Yellow Devil's effectiveness at wearing him down prompted Wily to revise the robot in Mega Man 3 to make it sturdier and impervious to its preceding weakness, instead being weak to Hard Man's Hard Knuckle.
A variant created by the supercomputer Ra Moon to combat against Mega Man in Super Adventure Rockman. Also the first variant not constructed by Wily himself. Unlike the previous and future iterations, the New Yellow Devil was considerably more mobile, as it tended to jump around several feet forward or back. Its attacks consisted of an eye blast, a trio of red energy shots and pounding the ground with either or both its arms. While it had the ability to morph into a mobile puddle to guard against Mega Man's buster attacks, it did not display the traditional mass projectile attack. Despite its name, it was primarily black in color and featured arm and leg bands with ancient symbols lined across their surfaces along with a collar with a similar design, making its design more detailed than most other Devils. Due to its resemblance to creator Ra Moon, it has been dubbed by fans (and in the Archie Comics adaptation) the "Ra Devil".
Yellow Devil MK-III
Much later, Wily revisited the Yellow Devil's design once more to create an even more powerful version that appeared in Mega Man 11, yet again rendering it more durable and immune to its previous weakness, now weak to Blast Man's Chain Blast. This version was also enhanced with a Speed Gear to further increase its attacking power and give it new abilities.
This heavily modified variant of the Yellow Devil first appears in Mega Man 8 in the third Wily Tower stage, and then as the first boss of Mega Man & Bass as it was rebuilt by King after he stole data from the Robot Museum. Rather than being made out of a more solid substance, its main body is made out of something akin to gelatin and its eye is that of a normal color rather than the traditional red, and is located in the very center of the body mass. Due to this, the player must repeatedly shoot at the Green Devil to create an opening in its body in order to expose and shoot the eye. It can attack not only by flinging pieces of itself at Mega Man, but also by seeping through the grates on the floor in order to try and attack Mega Man from below. The Mega Man & Bass version is much weaker, losing all its mobility and most of its attacks. It stays in one spot the entire time and can only attack by flinging pieces of itself at Mega Man or Bass, which will either bounce or roll across the floor. Curiously, the Green Devil never gets any smaller despite the fact that it's almost always throwing pieces of itself at its opponent.
A servant of Terra, leader of the Stardroids in Mega Man V, sent to destroy Mega Man after four of his brethren had been felled in combat. He was considerably larger than the other Devils up to this point, as his vertical columns consisted of five blocks rather than the traditional four. While he shares the same mass projectile attacks as most other Devils, he bounces the top two down due to his sheer size.
The Petit Devils, seen in Mega Man 9, are unique to the Devil series in that they are the only ones that appear as normal enemies rather than fortress bosses meant to defend Wily's castle. They also look and behave much differently to their larger cousins; Petit Devils are small, limbless robots that simply hover over a small area and split into several smaller versions of themselves when destroyed. They're also the first known type of Devil to have been seen in a Robot Master's stage, as they appear abundantly in Galaxy Man's stage.
A more unique variation from Mega Man 9. Instead of a humanoid creature, the Twin Devil was actually two identical shapeless models that worked in tandem. Possessing the ability of flight, they have much dominion of the air and they attack by switching places, shooting pieces of themselves to each other's relative location until they've completely reformed and can adjust their height afterwards.
Another unorthodox variant from Mega Man 10. Unlike the other models, as its name indicates, it is blocky in nature and its pieces can wedge themselves into the occupying room's gaps. It situates itself in the center of the room rather than in a corner. When fully formed, however, it fires plasma beams to attack.
A variant from Rockman & Forte: Mirai Kara no Chousensha, this model was mostly immobile and its attack consisted of sliding three balls of goo across the floor before sliding to the other side of the room. It's eye pops in and out of its core, and is only vulnerable when it is out.
A mysterious creation most likely created by the Zero Virus that appears in Mega Man X5 within the confines of the virus area X and Zero investigate. It is a much sturdier model than other Devil models and also inflicts far more damage than its predecessors. While it continues the mass projectile tradition, it instead breaks its body up into a 4x4 column of pieces that is surrounded by an outline of its normal form that inflicts a great amount of damage if touched. Even after it has reformed, it will take some time adjusting, as it has the ability to move its eye's position to any part of its body. Its eye can also fire off a projectile. It also has the ability to further change its form, morphing itself into the Wily Machine 6 to cover more ground and therefore making itself more dangerous.
Even after a couple of centuries, the Devil design remained alive in the form of the Rainbow Devil, fought in the first Mega Man Zero as well as Mega Man Zero 2. Like the Green Devil, it was covered in an amorphous gelatin, but it was much more mobile and active with its attacks. Rather than splitting its mass into projectiles, it flung its pieces around by spinning in one place where the pieces would impact with a small explosion, but remained intact and would return to the main body. Also unlike the other Devils, it was controlled by a head rather than an eye, making it a rather special type of Pantheon.
The latest known model, from Mega Man ZX, yet again controlled by a head rather than an eye. However, what separated itself apart from others was the usage of magma rather than a specially-made substance. Its design was decidedly more demonic in nature and significantly more advanced, since it could control which parts of it were cooled down while the rest of it remained liquid. It often lurched towards enemies, or alternatively separated its mass into falling globs of lava before reconstructing a new body from the lava below.
This variant was developed by Drs. Eggman and Wily in the Archie Comics crossover Worlds Collide, fusing elements of Chaos with that of the Devil robots. It was virtually uncontrollable to anyone besides Eggman and Wily, and effectively acted as a dog while under their control.
A Devil model from the Mega Man comic by Dreamwave Productions.