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.
(..) Perhaps we should have done things a more interesting way. No, no, no. That is the fault of the poets, not you. And who is this? This is Torvi, my son Bjorn's companion. And this is Astrid, one of my shield-maidens. I leave you in their company. - We'll see. - Really? You had no right to free my (..)2017 TvShow: Vikings Title: The Great Army Season: 4 Episode 17