Yankee/yankii is the word used to describe... what would we call them in English? Juvenile delinquents? They'll usually have their hair dyed (badly), dress in deliberately unfashionable clothes (anti-fashion statement?), either chain smoke or chew gum, squat on the sidewalk or outside convenience stores or other gathering places, talk rough, get into fights, have snotfaced kids with mullet hairdoes (or worse). And that's just the girls! Actually the word is used for guys or girls.
Actually, this reminds me that Kersten could save herself (and her friend) a lot of time by getting the movie 下妻物語 (Shimotsuma Monogatari/Kamikaze Girls)
available from YesAsia, which is about the friendship between a lolita girl and a yankii. Perfect research material. And it stars two of the most popular actresses of the day, Fukuda Kyoko and Tsuchiya Anna.
This
page of manga art has a yankii girl image (bottom pic).
For info on the origins of the word in Japanese, see this
ンキー_(不良少年)]Wikipedia page. You'll often see people on TV described as 元不良 or 元ヤンキー, meaning they are reformed delinquents. They're the only ones who can "get through" to the troubled youth of today. Blah, blah, blah.
This guy is one example (they even call him Yankii-sensei!).