blabmouth

Example:He is quite a mum and always keeps his thoughts to himself.

Definition:Silent; not speaking or talking.

Example:She is very reserved and doesn't often share her personal thoughts.

Definition:Not habitually disposed to disclose one's thoughts, feelings, or knowledge easily.

Example:He preferred to sit quietly and not talk too much.

Definition:Silent, or calm and unexcited; without noise or disturbance.