Rouge the Bat
Rouge the Bat made her debut in Sonic Adventure 2 and has starred in other Sonic the Hedgehog video games ever since.
She is 17 years of age. She basically has the same attacks as Knuckles, except she specializes in kicking. Rouge is treasure hunter who thinks all the world's gems belong to her. She is also a government spy sent out to collect information on the "ultimate life form".
Some suspect that she has fallen in love with Knuckles in the video games. More suspect that the feeling is mutual.
She has appeared in games including Sonic Adventure 2/Battle, Sonic Battle, Sonic Heroes (where she leads Team Dark) and in the Sonic the Hedgehog Comics.
Rouge is vain and selfish, and cares about nothing more than her and her gems. I personally despise that trait, but it is part of her character and I will not abuse it. Many say that she and Shadow the Hedgehog have a connection, but merely exists in fanfiction.
Overall, she would do something if the world were in trouble, which is why she is not always considered a "bad guy".