I feel like I'm writing this one about myself. I consider myself the wall flower at parties.
So who is the wall flower (other than some rock band, apparently)? According to dictionary.com, a wall flower is:
To me, a wallflower is the person who doesn't join in, is shy, and unpopular.
So let's talk about the wallflower in a book. They're the person who's overlooked in a lot of writing, because, well, they're shy and boring. What does the wallflower think of this? It makes them angry but they don't think they can change their circumstances. They feel stuck being in the unpopular crowd, always feeling jealous of those who are in the 'know.'
As jealous as they are, they make no attempts to go outside their comfort zone to get to know the popular group. They'd rather just sit by the sidelines and not be noticed than to risk being made fun of or failing.
So when writing about a wallflower, make him or her step outside their comfort zone and be in the limelight. That'll ramp up the conflict in your story and even make the reader relate to not being in the popular group. Because, face it, we're not all with the in-crowd all the time. So everyone can relate.
Have a great day!