HomeSonic has a Mom who is a queen. On top of that he has a brother and a sister. Yep, Sonic is part of a set of triplets this time around. So Sonic is born and he's got a Mom a brother and a sister. Then Robotnik takes over the prosperous city of Mobotropolis and the queen is forced to flee to avoid capture, she also takes her children with her. But the oracle of Delphius, whom you can read about on the cast of SU page, tells Queen Aleena, which is the name of Sonic's Mom, that she must leave her children behind in Mobotropolis and one day she will be reunited with them and they will destroy Dr. Robotnik together. Queen Aleena was hard pressed to leave her children behind but she knew it was the for good of everyone. So she leaves her children behind at different peoples homes. Sonic gets left with some mountaineers, Manic [brother] get lefts with a nice old lady, but before the nice lady can come and get the child he was stolen by a thief named Farrel. Sonia [sister] was left with a rich aristocrat (Robotnik didn't roboticize them because he needed the aristocrats to fund his operation) to live with. When Sonic had been living with the mountaineers for a little while they were roboticized, though, and he went to live with his Uncle Chuck.
Okay so it has been about 15 years. All of them has grown good at different instruments. Sonic-guitar, Manic-Drums and Sonia-Piano. We'll get back to that later. So Sonic is visited by the Oracle of Delphius and told that he is a prince and about his siblings. Later on he happens to meet all of them at a club. They realize that they are siblings. So the trio is finally together. In the mean time Buttnik has roboticized all of their guardians. The trio are furious and excruciatingly sad but they know that there is only one thing they can do, and that is find their mother so that the group of 4 will be complete and they can kick Buttnik butt. AND THAT IS HOW SONIC UNDERGROUND BEGINS!