In this series the mood is very lighthearted, and the main characters are Sonic, Tails, Scratch (a robot rooster created to destroy Sonic), Grounder (his little brother), and the evil Dr. Robotnik. The planet of Mobius is bright, colorful, and spacious, and situations generally work out for the best.
In contrast to Adventures of Sonic the Hedgehog, this show is dark and brooding. The main characters include Sonic, Tails, Princess Sally Acorn and her computer NICOLE, Antoine, Bunny Rabbot, Rotor, Dulcy, Dr. Robotnik, and his nephew Sniveley. Robotnik has turned the city of Mobotropolis into a disgusting heap of metal, renaming it Robotropolis and turning whatever inhabitants didn't escape into robots. Sonic and his friends live out in Knothole Village, in the forest, where they undermine Robotnik at every opportunity.
This series most closely resembled Sonic the Hedgehog, gloomy and dark. The main characters are Sonic, his sister Sonia, his brother Manic, Dr. Robotnik, and his assistants Sleet and Dingo. Sonic, Sonia, and Manic were born to Queen Aleena as a princess and two princes before Robotnik overtook the city. Queen Aleena had been warned through prophecy that this would happen and distributed her children to trusted caretakers, where they were safe until the day they would take back the throne. They "plot and plan Robotnik's overthrow" while spending time on their band, the Sonic Underground.
The Sonic X series has a more lighthearted feel to it while not appealing exclusively to children like Adventures. The cast is large, including Sonic, Tails, Knuckles, Amy Rose, Shadow, Rogue, Cream and Cheese, Eggman and his assistants Decoe and Bocoe, Chris Thorndyke and his family, friends, and servants. Sonic and his friends were transported to a strange, parallel world while fighting Eggman for control of the Chaos Emeralds, and the adjustment isn't eased by Eggman's also being transported into this other dimension! Action and hilarity ensue.
The Fastest Thing Alive
Sonic the Hedgehog is a blue hedgehog who can run at supersonic speed, fighting the evil Dr. Robotnik (or Eggman) for control of planet Mobius. With allies like Tails and Amy Rose, rivals like Knuckles the Echidna and Shadow the Hedgehog, and enemies like Dr. Robotnik and Metal Sonic, Sonic's supporting cast is large and growing with each new game! Originally having his start in the Sonic the Hedgehog video game series with 1991's Sonic the Hedgehog, Sonic has gone on to become Sega Games' official mascot, the world's most popular video game character, and star of comic books, a movie, and television shows based off the original Sega games.