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]

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