Formal:
* Taciturn: Reserved or uncommunicative in speech; saying little.
* Reticent: Inclined to be silent or secretive; uncommunicative.
* Introverted: Characterized by or tending towards introversion, i.e. a focus on one's inner world and thoughts.
* Quiet: Making little or no noise.
* Reserved: Slow to reveal emotion or opinions.
Informal:
* Quiet: (Simple and straightforward)
* Shy: Nervous or timid in the company of other people.
* Soft-spoken: Speaking quietly and gently.
* Laid-back: Relaxed and easygoing; not inclined to talk much.
* Unsociable: Not enjoying the company of other people.
* Wallflower: A shy or retiring person, especially one who is overlooked at social gatherings.
The best word will depend on the specific context and the person's personality.