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Dr. Robotnik/Eggman The self-proclaimed genius is always plotting to take over the world. He's right - he's an IQ of over 300. He can build machines ranging from mechs to planes - not unlike Tails. He is a bit nasty but has respect for those who believe the same as him.
(First appearance: Sonic the Hedgehog)

Shadow the Hedgehog He could be Sonic's alter-ego. He's exactly the same appearance-wise, but he's black with red streaks. He is very mysterious and doesn't show his emotions easily. He is very clever, and has visions involving Maria Robotnik. He was created by Professor Gerald.
(First appearance: Sonic Adventure 2)

Rouge the Bat This bat is crazy about jewels. "All the world's gems are mine (Rouge's) to keep!" Unfortunately for Knuckles, this includes the Master Emerald, so it's a race to find the pieces! Neither Eggman or Shadow know her true purpose, but she is looking for information on the 'Ultimate Life Form'...
(First appearance: Sonic Adventure 2)

Professor Gerald Robotnik He is the grandfather of Dr. Eggman. He created the 'Ultimate Life Form', but disappeared soon afterwards in mysterious circumstances.
(First appearance: Sonic Adventure 2)

Chaos Chaos is a liquid creature that thrives on Chaos Emeralds. Every time Eggman 'feeds' him one, he transforms into a more powerful Chaos. It's only a matter of time before he gets them all...
(First appearance: Sonic Adventure)

Metal Sonic, Mech Knuckles, Tails Doll These are versions of Sonic, Knuckles and Tails created by Eggman. While they have special features, they are not up to the standard of the originals.
(Metal Sonic's first appearance: Sonic the Hedgehog 2)