demure
Meaning: quiet, modest, reserved
Usage: Though everyone else at the party was dancing and going crazy, she remained demure.
diffident
Meaning: shy, quiet, modest
Usage: While eating dinner with the adults, the diffident youth did not speak for fear of seeming presumptuous.
staid
Meaning: sedate, serious, self-restrained
Usage: The staid butler never changed his expression no matter what happened.