As far as I know, at first there were the onee-gals - described above. Then some Shibuya boys started a style to match - onii (older brother, to the onee - "older sister"). And then the gals started copying that boys' style, and it became onii for gals. It's inspired by what you'd see in "Men's egg" etc, a sleek look with somewhat macho elements. (Often you add some feminine accessories like heels to make it look less like crossdressing, but some play actively with the style.)This is actually a style for women or men, so you can guess it's quite generally. On a man, it tends to mean a classier version of Gyaru-oh, leaving behind the brightly coloured hair and clothes of their youth in exchange for plenty of designer brands like D&G, dressing mostly in jeans with tank tops and shirts, of course with some very stylish, smart shoes or boots. Most Onii-kei guys tend to have sunglasses permanently attached to their face if they're not in a club. Obviously cool factor is very, very important to them. On a girl, Onii-Kei basically means something plaid (a shirt or shorts), or a tribal print t-shirt worn with jeans/shorts. Really just a mannish Gyaru style, although some have been known to follow onii-kei exactly as men do.