Sonic the Hedgehog or Underground Sonic is the native version of Sonic in the Sonic Underground zone. Although he has a few similarities to the Prime Sonic, his backstory mostly differs in his family; possessing two siblings named Sonia and Manic as well as his missing mother, Queen Aleena. In the Underground Zone, Sonic and his siblings are part of the underground resistance against Dr. Robotnik as well as a musical band; he is also an heir to the throne of this version's Mobius.
Before Giant Borg
Shortly after Sonic, Manic, and Sonia’s births, the Underground Zone’s local Dr. Robotnik would seize power of Mobotropolis and convert it into the polluted and metallic city of Robotropolis. A bounty was placed on the children of the queen, and thus Aleena was forced to take them into hiding. Eventually, the Oracle of Delphius appeared before her with a prophesy which foretold that she would have to separate with her children in order save they’re lives. Sending the siblings to three different caretakers, Sonic was taken in by his Uncle Chuck. Sonic would learn how to play electric guitar and went off to form a band after meeting Sonia and Manic. The Oracle of Delphius reappeared to tell the children of they’re origins. Shortly afterwards, Dr. Robotnik captured Uncle Chuck, as well as Sonia and Manic’s guardians and roboticized them. It was then that the three hedgehog children would form an alliance with each other to locate they’re missing mother. (SSS: #10)
Giant Borg’s Attack
When Robo-Robotnik utilized the Giant Borg, Zonic the Zone Cop would seek assistance from each version of Sonic across the Multiverse. Yet because of the overwhelming amount of Zones in the Multiverse, the Underground Zone and Underground Sonic was overlooked. Because of this, the Underground Sonic never received a piece of the Giant Borg when they were distributed to all the Sonics across the Multiverse. (StH: #19, SSS: #10)
Evil Sonic would appear in the Underground Zone on a mission from Robo-Robotnik to recover the Giant Borg. Believing Underground Sonic also possessed a piece of the Giant Borg; he located and interrogated him and his siblings. The evil hedgehog departed only for two new Sonics to appear: Prime Sonic and Zonic. Underground Sonic, Sonia, and Manic mistook Prime Sonic for Evil Sonic until the two new hedgehogs explained who they are. Zonic would remark how he had a lack of data on Underground Sonic and the Sonic Underground dimension and would request an explanation of the Zone’s history. (SSS: #10)
Underground Robotnik appeared in the Giant Borg after recovering the pieces from Evil Sonic. Believing he captured Underground Sonic (after mistaking Evil Sonic for him), he was hoping to find Sonia, Manic, and the rest of they’re resistance to destroy them once and for all. Sonia, Manic, Zonic, and the Sonics formulated a plan to defeat him. Using himself as bait, Underground Sonic used his speed to distract the Underground Robotnik as Prime Sonic and Zonic would climb into the Giant Borg to defeat him. The Giant Borg fired a heat-seeking missile at Underground Sonic which he decided to use to his advantage, running at the Giant Borg as Prime Sonic and Zonic took care of Dr. Robotnik. The missile would destroy the Giant Borg weapon once and for all. (SSS: #10)
After the battle, Zonic and the Sonics followed Manic and Sonia into Robotnik’s prison where Evil Sonic was located. They would break Evil Sonic out of jail who remarked that they were just arresting him again, which Prime Sonic would point out that it still beats being roboticized. Prime Sonic and Zonic thanked Underground Sonic and his brother and sister before taking Evil Sonic and leaving the Underground Zone. Sonic, Sonia, and Manic were then left to speculate whether they would ever see Prime Sonic and Zonic ever again. (SSS: #10)