Courtney Whitmore aka Stargirl
The character of Stargirl falls into her own category. I wasn't really aware of her until 2005 or so and besides her main series, Stars and S.T.R.I.P.E., I never "followed" her per se. She does appear in other comics I own, but she is not a character I seek out.
She eventually made it onto the Justice League cartoon and the show Smallville.
I am including Stargirl for one big reason. I love how she was "created".