Companion

Noun

Template:Countable A companion is somebody who you spend time with, usually because you enjoy being with them. Template:Synonyms
In some places, it's best to travel with a companion.

Because she is blind, the dog is her constant companion.
Template:Countable If $x$ is a companion to $y$ , $x$ and $y$ go together. Template:Synonyms
Pain is a natural companion to art.

Scientists noticed that this normal star circles an unseen companion.