Dr. Robotnik

Dr. Eggman is a tyranical megalomaniac with an IQ of 300 and the archnemesis of the Freedom Fighters. He claims that his real name is Ivo Robotnik, and that "Eggman" is just a nickname the Freedom Fighters gave him.

Appearance
Dr. Eggman is depicted as being rotund in appearance, with black pants and leggings, white gloves, a bald head, pince-nez glasses with blue lenses, a bright red nose, and a very bushy auburn moustache. He also wears a red shirt with an oversized yellow collar and gray buckles on his pants.

Before the Super Genesis Wave
Eggman was the sole character to not appear in "The Sonic Saturday Morning Cartoon", since his history is identical with his counterpart from Wendy O. Koopa's comics, who came from another timeline. When he unleashed the Super Genesis Wave, he merged with his cartoon counterpart...

After the Super Genesis Wave
Not much is known about Eggman's history, only that he once was the Acorn Kingdom's warlord once. Bowser, however, never trusted him. In 3225 Eggman took over Mobotropolis, turning it into Robotropolis. Bowser tried to stop him and even was strong-willed enough to resist Eggman's robotization, but was sealed in the Null Space right afterwards. Eggman has been Sonic the Hedgehog's archnemesis ever since.

Personality
Eggman is the ruthless and megalomaniacal ruler of the Eggman Empire. He is a power-hungry tyrant who believes that his superior intellect makes him the rightful ruler of the world, a view which makes him incredibly arrogant and gives him a psychotic sense of self-entitlement that urges him to not just rule over the planet, but over the entire universe as well. Eggman is a strict man of science and technology, and has little care for the environment, magic, plant life or other living creatures, preferring to have his territories be completely dominated by factories and machinery with no sign of natural elements. His ultimate goal is to turn the world into his ideal robotic empire which he has dubbed "Eggmanland" and is determined to cause as much chaos and destruction to achieve this goal.

He is a manipulative individual who will do whatever it takes to further his plans and will gladly take control over the minds of Mobians, Overlanders or even divine beings against their will all for the sake of achieving his goals without caring about the harm he does to them or the world. His uncaring personality is mostly due to his over-inflated ego, as he only views himself and his machinery with utmost pride; the only other individual Eggman has ever admired is his grandfather Gerald Robotnik, whose life would play a major role in influencing Eggman's views. Eggman has no problem boasting about his superior intellect and mastery of science, often making long-winded proclamations about his superiority and looking down on anyone who would disagree. While Eggman's overly prideful views are the core parts of his personality, they are also his greatest weakness, as they cause him to become far too overconfident and arrogant when facing those who challenge his attempts at world domination and it usually leads to his downfall.

Eggman's psychotic mentality makes him a very dangerous man, but it also seems to make him somewhat eccentric and he often acts like an immature and short-tempered blowhard, especially when things don't go his way. Nonetheless, he is still a cruel and conniving individual under the right circumstances, and when he has complete control over the situation, and his plans go accordingly he prefers to toy with his enemies rather than finish them off from the start, but this arrogance as mentioned before is what usually leads to his own downfall.

Powers and abilities
Eggman is a brilliant scientist and inventor with an IQ of 300 whose mastery of technology knows no bounds, able to create machines that can cause mass destruction or harness the power of nature and the divine itself. His remarkable skills as an engineer and programmer are best evidenced by the armies of robots and mechs he's created for his numerous world domination attempts, some of which he has been able to build and program in only a matter of hours. He is an incredibly skilled pilot: able to manipulate many different kinds of his own mechs, robots and vehicles in difficult battles with Sonic. His most well known ability is his masterminded plots and schemes, however even when successful his plans usually meet with failure due to Sonic and his friends and Eggman's own arrogant overconfidence.

Sonic the Hedgehog
Sonic is Eggman's arch nemesis, due to the latter having constantly foiled his schemes for world domination.

While Eggman will stop at nothing to defeat Sonic, and vice versa, Sonic has never really been shown to take much account of his adversary; he takes his fights with Eggman and his goons very lightly, cracking jokes and remaining fully confident that he can ruin his enemy's day, even when being confronted by the madman's deadliest of forces. The Merger Magikoopa intends to make Sonic start taking the fights more seriously after Operation:Endgame, as he "can't afford not to care anymore".

Sally Acorn
Eggman has a secret desire to turn Sally into his primest robot henchwoman, by completely destroying her soul.

Bowser Jr. and Metallix the Metal Sonic
Eggman hates Bowser Jr. for using technology to help the Freedom Fighters and for calling him a coward, and hates the fact Metallix is free willed. How does Eggman knows that Metallix is aiming to gain a soul is unknown, as Metallix only knows about Eggman that he must die.

Trivia

 * According to Roy Koopa, Eggman's real first name is Julian.
 * Eggman seems to like portraying Sonic as a jerk, as he has friends that do so in something named "YouTube Poops" and that, for that reason, he hadn't roboticized.
 * According to William Afton, Robotnik is a cocksucker.