Someone who isn't necessarily shy, but never really tells you a lot about themselves. They observe almost everything and listen to everything you have to say without criticizing or judging. These people are often the most sincere, kind, and wonderfully interesting people, yet fail to be attractive to the opposite sex for some reason.
I thought that she was mean and stand-offish at first, but then I got to know her and found out that she's really a somewhat shy, beautiful soul. She's a bit of a wallflower if you ask me.