Doctor Eggman

Doctor Ivo Robotnik, better known by his nickname Doctor Eggman , is Sonic the Hedgehog's arch-enemy and therefore, the main antagonist of the  franchise. Eggman is portrayed within the series as an obese mad scientist with the desire of world domination to create his "Eggman Empire", though his plans are mostly foiled by Sonic and his friends. He briefly appears as a minor antagonist in McLeodGaming's Flash cartoon series, A Super Mario World.

Character description
In April 1990, SEGA commissioned its AM8 R&D department to create a character who would replace Alex Kidd as the company's mascot, as well as compete against Nintendo's flagship character, Mario. The idea of an egg-shaped character became the basis of the visual design for Eggman. In creating the "bad guy" for the Sonic series, the development team wanted a character who was "the opposite of Sonic;" a character who represented "machinery" and "development" to play on the then-growing debate between developers and environmentalists.

The English instruction manual for his debut game  described the character's full name as "Doctor Ivo Robotnik", while the original Japanese version's instruction manual for the same game called him "Doctor Eggman". It was not until 1999's  that the character was called both "Eggman" and "Robotnik" in the English version, with all following English releases to date referring to him as "Doctor Eggman".

Eggman is described as being a certifiable genius with an IQ of 300. His fondness for machines has also made him a renown authority on robotics. Ultimately, his goal is to conquer the world and create his ultimate "utopia", the Eggman Empire (alternatively known as the Robotnik Empire, Eggmanland, Robotnikland, or Robotropolis). He selfishly never gives up on this matter, and does not care for others' opinions. He considers those who would interrupt his plans a prime threat. His abominable laughter and maniacal declarations contrast his self-professed softer side. Although Sonic has always ruined his evil plans, Eggman begrudgingly holds a secret respect for his determination.

In A Super Mario World
Doctor Eggman is one of the minor antagonists in A Super Mario World. He is briefly seen in the second episode riding his Egg Mobile, and he reveals that he aided Bowser to build their "Bowser-Eggman Cruiser" (a flying Bowser Castle-esque mechanized unit) after Mario, Link and Cloud Strife confront Bowser on said castle's roof. Eggman abandons Bowser after he realizes his work is done and is no longer seen throughout the remainder of the series. Because of this, he is the only antagonist of the series that did not get killed by the heroes.

Oddly, Eggman technically worked alongside his arch-enemy Sonic the Hedgehog and Knuckles the Echidna, who acted as Bowser's guardians. It is unknown whether any of these characters were aware of this.

Trivia

 * Although Dr. Eggman does not physically appear in the  series, he makes a minor cameo in Super Smash Flash 2 during one of ' victory poses when Tails spins the signpost from the side with Eggman's face to the side with his face instead. He is also the basis for one of Tails' costumes, and he is mentioned in event 46: Eggman Empire.