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Sonic the Hedgehog He's the fastest thing alive, no question. He is a bit impatient, but deep down is really friendly. His best friend is Miles "Tails" Prower (see below), and he has a rocky relationship with Knuckles. He also has to stand Amy's constant obsession with Sonic. Time after time, no matter how sticky the situation, Sonic always squeezes out of trouble.
(First appearance: Sonic the Hedgehog)

Miles "Tails" Prower the Fox (my favourite character) This fox boy with two tails can use his 'deformation' to do what no other fox can: fly. He can also create machines to rival Eggman's. He is Sonic's best friend and is seldom seen to be far away from him.
(First appearance: Sonic the Hedgehog 2)

Knuckles the Echidna The red Sonic-clone is the protecter of the Master Emerald and is fiercely protective of his work. His major downfall is that he puts his trust in others too easily, and as a result is very gullible. He makes up for it though with his hard punch.
(First appearance: Sonic & Knuckles)

Amy Rose the Hedgehog She is madly obsessive about Sonic, and sometimes it leads her into trouble. She is really warm-heated however, and is very close to her friends.
(First appearance: Sonic CD)

Big the Cat He is a cat that loves to fish. His best friend is Froggy, and he goes everywhere with him. When Froggy is lost Big does everything he can to get him back. He's not very clever and is a bit simple, but it's hard not to like him.
(First appearance: Sonic Adventure)

E-102 Gamma E-102 was created by Dr. Eggman to be part of the 'E Series'. However, he disobeys his master to join up with Sonic and friends, thanks to Amy. He goes on a mission to free all his 'E' friends, as they're really animals but into machines.
(First appearance: Sonic Adventure)

Tikal the Echidna She is from the past and trapped Chaos in the Master Emerald. She is also a great friend of Chao (not Chaos) and likes to talk to them. In Sonic Adventure, Sonic and friends have visions involving her, usually at the most inconveinent times :)
(First appearance: Sonic Adventure)

Super Sonic/Super Tails/Hyper Knuckles These are Sonic, Tails and Knuckles after collecting all 7 Chaos Emeralds and 50 rings. All their abilities are souped up when in this form.
(First appearance: Sonic the Hedgehog 2 (unless anyone knows differently))