(n.) One who lies with another in the same bed; a person who shares one's couch.

ace adjunct ally amigo associate bedmate bosom buddy brother brother-in-arms buddy bunkie bunkmate butty camarade chamberfellow chum classmate coadjutor cohort colleague comate companion company compatriot compeer comrade confederate confrere consociate consort copartner crony fellow fellow member fellow student girl friend gossip mate messmate old crony pal pard pardner partner playfellow playmate roommate schoolfellow schoolmate shipmate side partner sidekick teammate workfellow yokefellow yokemate