"She asked you" means that she made an inquiry of you. She asked you a direct question - like, "What time is it?" or "Did you see The Walking Dead last night on TV?"
"She asked for you" means that you were not immediately available and that she asked someone else to find you for her, or inquired as to your whereabouts. For example, at a clothing store sales counter: "Is Mary available? I spoke to her on the phone yesterday and she told me that she'd set aside one of those half-price designer sweaters for me." Or during a conversation among a group of coffee shop co-workers: "Hey, Bob - remember that really hot blonde that came in here last Friday? The one we were all flirting with? Well, she stopped by yesterday and she asked for you." Bob (not quite believing the story): "No way. Really?" "Really! I asked 'May I help you?' and she looked disappointed and said 'Isn't Bob working today?'"
Student or Learner