The old dog had been her 'constant companion' for over 12 years. [=had been with her most of the time for 12 years]
The book is a 'companion to' the television series with the same name.
The book is the 'companion' volume 'to' his previous book on jazz.
(..) father Fanch Tanguy. Isabella Pigot. Fanch Tanguy. That's just what we need, a Briton. In this whole mess. Thanks for the coffee. Your companion, Tarpon, is he real? Yes. - You can never see him. - He's very modest. There's something else I wanted to tell you, but I can't remember. (..)1981 To Kill a Cop
(..) a check madam Pigot! Then I'll ask you the same questions you have already answered at the police. Thank you monsieur. Fill it out in the name of Tarpon. Ogien Tarpon, my companion. You bring me the medical certificate when you have it. Great. There. Thank you, monsieur Jude. (..)1981 To Kill a Cop