WordNet (r) 3.0 (2006):

hard core
    n 1: the most dedicated and intensely loyal nucleus of a group
         or movement

Moby Thesaurus II by Grady Ward, 1.0:

49 Moby Thesaurus words for "hard core":
   adamant, adamantine, ardent, cast-iron, dour, fanatic, fiery, firm,
   flaming, flinty, grim, hard, heated, hot, hotheaded, immovable,
   immutable, implacable, impliable, inelastic, inexorable, inflamed,
   inflexible, intransigent, iron, irreconcilable, madcap, passionate,
   red-hot, relentless, rigid, rigorous, rock-ribbed, scorching,
   steely, stern, stiff, totally committed, unaffected, unalterable,
   unbending, unchangeable, uncompromising, ungiving, unmoved,
   unrelenting, unyielding, white-hot, zealous