What does it mean to be a wallflower? How is Charlie symbolic of a wallflower?

Wallflower: is someone with an introverted personality type (or in more extreme cases, social anxiety) who will attend parties and social gatherings, but will usually distance themselves from the crowd and actively avoid being in the limelight

How is Charlie symbolic of a wallflower:  

Explanation:

 How is Charlie symbolic of a wallflower:  Patrick does not use the word as an insult or a derogatory nickname. Instead, he calls Charlie a wallflower as a term of endearment. Patrick calls Charlie a wallflower during the party that Patrick, Sam, and Charlie go to after the homecoming game.

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