unassertive

from WordNet (r) 3.0 (2006)

unassertive
    adj 1: inclined to timidity or lack of self-confidence; "a shy
           unassertive person" [ant: {assertive}, {self-asserting},
           {self-assertive}]

from Moby Thesaurus II by Grady Ward, 1.0

34 Moby Thesaurus words for "unassertive":
      abject, accepting, acquiescent, agreeable, assenting, backward,
      bashful, complaisant, compliable, compliant, complying, consenting,
      diffident, modest, nondissenting, nonresistant, nonresisting,
      nonresistive, obedient, passive, rabbity, resigned, retiring,
      self-effacing, servile, shy, submissive, subservient, supine,
      timid, unassured, uncomplaining, unresistant, unresisting