The Collaborative International Dictionary of English v.0.48:

Unrepentant \Unrepentant\
   See repentant.

WordNet (r) 3.0 (2006):

unrepentant
    adj 1: not penitent or remorseful [syn: impenitent,
           unrepentant, unremorseful] [ant: penitent,
           repentant]
    2: stubbornly persistent in wrongdoing [syn: cussed,
       obdurate, obstinate, unrepentant]

Moby Thesaurus II by Grady Ward, 1.0:

23 Moby Thesaurus words for "unrepentant":
   callous, hard, hardened, impenitent, insolent, obdurate,
   recidivist, recidivistic, recidivous, regretless, remorseless,
   unabject, unashamed, uncontrite, unembarrassed, unmelted,
   unregenerate, unregretful, unremorseful, unrepenting, unsoftened,
   unsorry, untouched