WordNet (r) 3.0 (2006):

sidekick
    n 1: a close friend who accompanies his buddies in their
         activities [syn: buddy, brother, chum, crony,
         pal, sidekick]

Moby Thesaurus II by Grady Ward, 1.0:

68 Moby Thesaurus words for "sidekick":
   ace, amigo, associate, bedfellow, bedmate, birthmate, bosom buddy,
   buddy, bunkie, bunkmate, butty, camarade, chamberfellow, chum,
   classmate, clubmate, colleague, comate, companion, company,
   compeer, comrade, confrere, consociate, consort, copartner,
   copemate, copesmate, couchmate, cradlemate, crony, cupmate, fellow,
   fellow student, general partner, girl friend, gossip, jailmate,
   mate, messmate, old crony, pal, pard, pardner, partner, pewmate,
   playfellow, playmate, roommate, schoolfellow, schoolmate,
   secret partner, shelfmate, shipmate, shopmate, side partner,
   sidekicker, silent partner, sleeping partner, special partner,
   tablemate, teammate, tentmate, watchmate, waymate, workfellow,
   yokefellow, yokemate