It's over, hedgehogs! The prophecy will never come true!
— Dr. Robotnik, Sonic Underground
Robotnik has many different design changes. First off, he has a longer and more colored mustache then in the normal games. He also has a roboticized arm after an incident that happened to him in the Underground. He wears a long belt going around his waist. His boots are shorter and more colorful. Finally, he sports a cape, supported by big, heavy shoulder pads. Dr. Robotnik was modified specifically to look like he were a mechanic.
Dr. Robotnik began his rule by seizing the great city of Mobotropolis from the current monarch, Queen Aleena, and had it converted into his capital city: Robotropolis. After a bounty was placed on the newborn children of the queen, she was forced to go into hiding. Meanwhile, Dr. Robotnik roboticized each citizen but keep the wealthy around to help fund his polluted factories. Queen Aleena was forced to separate with her offspring after hearing a prophesy from the Oracle of Delphius. Dr. Robotnik would have each of the children’s caretakers (Chuck, Lady Windermere and Farrell) captured and roboticized. Yet resistance was met in the form of the queen’s children, who were now grown. Sonic the Hedgehog, Sonia, and Manic, now part of a musical band would grow to be the biggest enemy in Dr. Robotnik’s rule since he came into power.
While Robotnik mostly stayed on the sidelines during the ongoing war with the resistance, one of the major things he did in the beginning was where he tried to destroy Sonic. However, it backfired when Sonic foiled his plans. He only captured Sonic successfully one time, Sonia a couple times, and Manic multiple times. One of the more famous incidents was where Sonia was faced with a choice to save either Manic or Sonic; if she refused, both would be subject to the Roboticizer.
Evil Sonic would come to invade Robotnik's dimension one day, seeking out pieces of the Giant Borg, only to be captured by Dingo and Sleet. When brought in, he would mistake Dr. Robotnik for an alternate version of him called Robo-Robotnik. Likewise, Dr. Robotnik would also mistake Evil Sonic for his version of Sonic. Evil Sonic would give him all the pieces of the Giant Borg, but Dr. Robotnik had him sent away anyway to be roboticized.
Dr. Robotnik would reassemble the Giant Borg and used it to seek out Sonia, Manic, and the rest of their resistance. To his surprise, Sonic would come out to challenge him, even though he thought he captured him. Using a heat-seeking-missile, Dr. Robotnik fired upon Sonic, only to be taken by surprised by two other versions of Sonic: Zonic the Zone Cop and Sonic Prime. Sonic would then turn the missile back at the Giant Borg, destroying it and ending Underground Robotnik’s rampage. He was last seen hanging from a pole above a building, mocked by the free citizens.
- The design of this version of Robotnik recycles the version of himself that appeared in Sonic the Hedgehog television series, though changing minor things such as changing the lower half of his outfit to black and completely covering the skin on his right bicep.
- Although the art of Robotnik supplied in this article displays him with yellow shoulderpads, they were red in the actual show, similar to that of the Robotnik from the Sonic the Hedgehog television series.
- "I'm declaring war on the resistance!"
- "Next time, Hedgehogs!"
- "Now, for starters; I'm going to close my eyes and count to 10. It'll make this finale very interesting, not for you, but, for me."
- "Mr. Sonic Hedgehog. Before the sun sets today, you too, will be walking down the aisle to the Roboticizer!"