Use 'wallflower' to describe someone quiet at social events, not necessarily unpopular. Although he was a wallflower at parties, his peers respected his intellect.
Pro Tip 2/3
Neutral Term
'Wallflower' is neither an insult nor a compliment; it's a neutral observation. Being a wallflower can be a choice, she just enjoyed observing.
Pro Tip 3/3
Personal Characteristic
'Wallflower' shows personality traits. Do not confuse with being temporarily alone. He wasn't lonely; as a wallflower, he preferred the quiet corner.