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.
(..) Mmm, brother, you look ravishing. It will hurt no less when I kill you in this form. Very well. Perhaps you prefer one of your new companions given that you seem to like them so much. Oh, this is much better. Ooh. The costume is a bit much. (..)2013 Thor: The Dark World