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.
(..) Do you remember anything about what happened? He has no memory of that day. - Curious. - Why are you here? Would your charming companion be so kind as to kneel? - My neck is beginning to hurt. - Kneel, Hodor. - Do you like to ride, Bran? - Yes. (..)2011 TvShow: Game of Thrones Title: Cripples, Bastards, and Broken Things Season: 1 Episode 4