WordNet sense 1 (temperamentally disinclined to talk):

untalkative

WordNet sense 2 (habitually reserved and uncommunicative):

taciturn

WordNet sense 3 (cool and formal in manner):

reserved, restrained, unemotional

WordNet sense 4 (reluctant to draw attention to yourself):

self-effacing, retiring

WordNet sense 5 (inclined to timidity or lack of self-confidence):

unassertive