wallflower; shy person at a party
Refers to someone who is too shy to join in social activities, especially at parties, and stays on the sidelines. Often used in the phrase 壁の花になる.
She was a wallflower at the party.
I ended up being a wallflower at my first group date.
Literally 'flower on the wall', a calque from English 'wallflower'. The expression describes someone who stands against the wall like a decoration, not participating in the dance or conversation.