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.
(..) Anyway,you need people of intelligence on this sort of... ...mission. Quest. Thing. Well,that rules you out,Pip. Nine companions. So be it. You shall be the Fellowship of the Ring. Great! Where are we going? (..)2001 The Lord of the Rings: The Fellowship of the Ring