sidekick (n.)

• ace
• amigo
• associate
• bedfellow
• buddy
• bunkmate
• butty
• chum
• classmate
• colleague
• companion
• company
• compeer
• comrade
• confrere
• consort
• copartner
• crony
• fellow
• gossip
• mate
• messmate
• pal
• pard
• pardner
• partner
• playfellow
• playmate
• roommate
• schoolfellow
• schoolmate
• shipmate
• tablemate
• teammate
• workfellow