Wallflower.

A wallflower is someone with an introverted personality type who will attend social gatherings, but will usually distance themselves from the crowd and actively avoid being in the limelight.
The name itself derives from the eponymous plant's unusual growth pattern against a wall as a stake or in cracks and gaps in stone walls. "Wallflowers" might literally stand against a wall and simply observe others at a social gathering, rather than mingle. This could be due to anxiety, shyness, lack of social skills or self-esteem.
So there you have it!  Thanks Wikipedia for explaining.