Formal:
* Reserved: This word implies a quiet and self-contained nature.
* Timid: This emphasizes a fear of being noticed or drawing attention.
* Unassuming: This suggests someone who is not self-promoting and avoids attention.
* Shy: This is a straightforward word for someone who is easily embarrassed or uncomfortable in social situations.
Informal:
* Wallflower: This refers to someone who is quiet and withdrawn, often at social gatherings.
* Shrinking violet: This is a more playful term for someone who is easily embarrassed and shy.
The best word to use will depend on the context and the nuance you want to convey.