Sonic Underground is an animated series that follows the adventures of Sonic the Hedgehog and his siblings, Sonia and Manic. The cartoon follows a main plot separate from all other Sonic the Hedgehog media. The series ran for one forty episode season. As of 11 June 2012, re-runs of the series began airing on Disney XD.
The show takes place in a separate canon and continuity than any other Sonic the Hedgehog media. Queen Aleena, the kind and fair ruler of Mobius, was overthrown by Dr. Robotnik and his robotic armies. Robotnik seized control of the planet and forced her into hiding. To preserve the dynasty, Queen Aleena separated and hid her three children: Sonic, Manic and Sonia after the Oracle of Delphius told her of a prophecy: one day they would reunite with their mother to form the "Council of Four", overthrow Robotnik, and become the rightful rulers of Mobius once more. Meanwhile, Dr. Robotnik did his best to set up an autocratic government, and legally turned anyone who stood against him in robots devoid of freewill, and forced the nobles into paying large amounts of money to him as tribute.
When Sonic, Manic and Sonia grew up, the Oracle of Delphius revealed the prophecy to them: So Sonic, Manic and Sonia decided to go on a quest, searching throughout Mobius for Queen Aleena. Dr. Robotnik, with the assistance of the bounty hunters Sleet and Dingo, tries constantly to capture the royal family and prevent the prophecy from being fulfilled.
The three siblings have powerful medallions that can change into musical instruments, and also weapons when the triplets are in perfect harmony with each other. Sonic's medallion is an electric guitar. Sonia's medallion is a keyboard that functions as a smoke machine, Manic's medallion is a drumset that can be used as an "earth controller" with cymbals that can deflect laserfire. All of the medallions can be used as laser guns. The three use the amulets not only to fight Robotnik's forces but to also as instruments for their underground (illegal) rock band, "Sonic Underground".
- Sonic The Hedgehog: The world's fastest hedgehog and leader of the Sonic Underground. He is the lead singer of Sonic Underground. His medallion's string is red. As fast as the speed of sound, Sonic can easily outrun danger. He is a big fan of chili dogs. He also has Aquaphobia, being terrified of water as he cannot swim. Sonic wears anti-gravity sneakers that allow him to hover over the ground while running. His voice was provided by Jaleel White, and his singing voice was provided by Sam Vincent.
- Sonia the Hedgehog: The only female member of the Sonic Underground and the sister of Sonic and Manic. Unlike her brothers, she was raised by an aristocratic foster family, giving her an upper class mentality. She hates getting dirty, is skilled at gymnastics and karate, and possesses superhuman strength, a photographic memory, and the ability to spin in a cyclonic manner similar to the Tasmanian Devil and Espio the Chameleon. Her medallion's string is yellow. Sonia has a pink motorcycle which can travel as fast as Sonic (it was destroyed in the 36th episode). Sonia develops an attraction towards Knuckles during the Flying Fortress saga of the series. She was also played by Jaleel White. Her singing voice was provided by Louise Vallance.
- Manic the Hedgehog: Manic is a cool and calculating hedgehog and was raised by thieves, and is a master of their trade. When Manic met his siblings, they disapproved of his thieving tendencies, but his skills as a lock-pick were greatly appreciated. Manic possesses neither Sonic's speed nor Sonia's spin-attack, thus making him the only member of the Sonic Underground who relies on entirely on his medallion for superpowers of his own. Manic is the drummer of the Sonic Underground. His medallion hides his drums, which can cause earthquakes; the Oracle of Delphius claims that his drums are the most powerful instruments of the three. His medallion's string is blue. Like Sonic and Sonia, he was voiced by Jaleel White. His singing voice was done by Tyley Ross.
- Queen Aleena Hedgehog: The mother of the Sonic Underground group and former ruler of Mobius before Robotnik's takeover. She had to give them up in order for the prophecy to become true. She spends the series on the run from her own children, until the time is right to reveal herself. Her character has many allies, including the Oracle of Delphius and Knuckles. She also does a bit of narrating in the start of every episode.
- Knuckles the Echidna: The guardian of the Master Emerald and the Floating Island. He is very protective of his island home, and has set many traps about the island. He has a pet dinosaur called Chomps. His great-grandfather Athair warns him that he must stay on the island in order to play his part in the freedom of Mobius. He was voiced by Ian James Corlett.
- Oracle of Delphius: An odd, wart-covered, reptilian anteater in a cloak. The Oracle prophesied that when Robotnik invaded Mobius, Aleena and her children would form the Council of Four to overthrow Robotnik and free Mobius. The Oracle lives in a cave somewhere in a cold region of Mobius, and is quite good at making chili dogs.
- Bartleby: A posh mink who is one of Robotropolis' richest aristocrats. Bartleby is Sonia's former fiancé. He dislikes her brothers and works as an agent for Robotnik. He speaks with an upper-class English accent.
- Cyrus: Cyrus is a lion and a technician for the Freedom Fighters. He is an old friend of Sonic. In his first appearance he was working as a spy for Robotnik, but quit upon discovering that the Freedom Fighters' Sanctuary was a hideout for their children. Cyrus's father was roboticized. His voice was also provided by Ian James Corlett.
- Dr. Robotnik: Robotnik is the ruler of Mobius, renamed Robotropolis under his rule, having conquered it with his army of Swatbots. He captures and roboticizes anyone who opposes him, transforming them into robotic servants. Robotnik's primary minions are Sleet and Dingo, two bounty hunters that will do anything to capture Sonic and his siblings. His voice was provided by Gary Chalk.
- Sleet: One of Robotnik's canine bounty hunters. This cunning wolf leads the bumbling Dingo about on missions to capture the hedgehogs. Sleet has a morphing device that can transform Dingo into anything Sleet wishes. Sleet is constantly being crushed by Dingo and is usually the one who suffers the most from their combined failures. Sleet was voiced by Maurice LaMarche.
- Dingo: One of Robotnik's canine bounty hunters. Dingo is a huge, muscle-bound blonde canid with an Australian accent, and is also very ignorant. He is partially roboticised in his hands and his left leg from the knee down, which is never explained. Dingo has a relentless crush on Sonia, but remains fiercely loyal to his partner, Sleet. Dingo was voiced by Peter Wilds