My DD is almost 16. She has always had a "tomboy" style. I sometimes hope that one day, forum member Elly will be her style mentor!
She wears jeans almost every day -- mostly Gap or Old Navy BF jeans, but she tends to wear them more like slouchy skinnies. Not super baggy. At home she wears sweat pants in winter, but she rarely if ever would wear those out of the house. In summer, she wears BF shorts. She doesn't care for leggings.
For tops, she wears plaid button front shirts, graphic tees with feminine drape, or loose cut boys' tees with fun patterns (from places like Out of Print). She also owns a few "dressy" blouses, which she wears with black trousers and a black jacket for a "le smoking" effect. In winter, she has some fun pullovers, some with graphic designs and some plain. She also wears a Roots hoodie and a vegan leather jacket. And of course a parka in winter!
Her footwear? Blundstone chisel toes which bookend her black hair. In summer she likes a pair of red Ecco sandals.
I wouldn't call her fashionable by any means. But I do think she dresses for herself and doesn't slavishly do what the other girls are doing.