Wallflower

from Merriam Webster Dictionary online

Main Entry:
wall·flow·er
Pronunciation:
\ˈwȯl-ˌflau̇(-ə)r\
Function:
noun
Date:
1578

1 a: any of several Old World perennial herbs (genus Cheiranthus) of the mustard family; especially : a hardy erect herb (C. cheiri) widely cultivated for its showy fragrant flowers
b: any of a related genus (Erysimum) of herbs with showy flowers

2 a: a person who from shyness or unpopularity remains on the sidelines of a social activity (as a dance)
b: a shy or reserved person