More informal than 'girl'? As in addressing someone? I don't think that such a word exists. Also nobody really calls each other "fellow" here; the word is used in contexts like "fellow traveler" (not what it sounds like, but instead a person who is not of a political classification but aids people of it anyways). Actually, off the top of my head, I'm not sure any other use of the word "fellow" sounds normal.

In America, we say "gal".