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.
(..) - Everyone walked out of languages. - Let's go to the X-ray... There's no X-ray today. The doctor's boiling jelly. And what if he took my graceful companion with him, beautiful Bacilisu? Who here has an invitation to go to Africa? No-one, we're just chewing the fat. (..)1989 Dva lidi v zoo