Doctor Robotnik is the main antagonist of Sonic Underground. He is the evil dictator over Mobius who is assisted in his schemes by Sleet and Dingo.
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 yellow shoulder pads. Dr. Robotnik was modified specifically to look like he were a mechanic.
Robotnik was a consistent villain throughout the series, but didn't appear in many of the episodes. One of the major things he did was in the beginning where he tried to destroy Sonic, but it then 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 episodes was where Sonia was faced with a choice to save either Manic or Sonic; if she refused, both would be subject to the Roboticizer.