olemus
Wiktionary:
1. a friend, acquaintance, or partner; someone with whom one spends time or accompanies
6. (figuratively) a thing or phenomenon that is closely associated with another thing, phenomenon, or person

companionate marriage -- prooviabielu
companion-in-arms -- võitluskaaslane
companionship -- kaaslus; partnerlus; selts
lifelong companion -- elukaaslane
travelling companion -- teekaaslane