A wallflower is a person who is introverted and is separate from the crowd, usually at a party. Not particularly shy but are reserved when surrounded by a lot of people and big personalities. Not really knowing what to say so can come across as socially awkward. This person will blend in and become almost invisible to others. They choose to stand back rather than getting involved as they prefer to observe. They have no desire to be the centre of attention.
Generally they are quite happy in themselves and accept being different. Wallflowers can be creative and wise.
A wall may be ordinary and unnoticeable but a flower is associated with beauty and can be rare, you won't truly learn until you get close.
He's a wallflower. You see things. You keep quiet about them. And you understand.
-Stephen Chbosky, The Perks of Being a Wallflower