JL8 (formerly Little League) is a webcomic based around the Justice League characters as young children in school.
In http://jl8comic.tumblr.com/post/26169864237/ who is the girl in the last panel meant to be? I recognize the rest of the characters so far, but this character hasn't been in any of the Justice League comics I have read so far (mainly Grant Morrison's run).
That is Karen Star, also known as Power Girl. The reason you don't know her, especially if you are reading Grant Morrison's Justice League, is she is NOT a member of the JLA.
She is a Kryptonian from a parallel universe (the Pre-Crisis Earth 2, to be precise) and she would be an analog of Supergirl from the mainstream DC Universe. She is gifted with all of the powers of mainstream Kryptonians with a bit of an anger management issue from time to time.
