A person who usually goes along unnoticed and doesn't talk much. On contrary to popular belief they don't hear and know everything (some hear and know less than usual) though some do. They are usually considered loners but are actually mostly shy or introverts. Wallflowers are also usually sincere and kind-hearted people who are misunderstood, and tend to be genuinely interesting once they open up and talk to you, though you'll probably need to talk to them first.
Random jock: Dude you know Lucas? Such a wallflower, he doesn't talk to anyone, what a loser.
Guy who talks to Lucas: You're a serious dick dude, and believe it or not, Lucas is the only one who doesn't think that, he said "I'm sure there's a reason he acts like that".
Random jock: Seriously? I fell kind of bad now...