companion; pal · partner; business associate

I don't have a partner.

To my chief strategist, David Axelrod, who's been a partner with me every step of the way. To the best campaign team ever assembled in the history of politics! You made this happen, and I am forever grateful for what you've sacrificed to get it done.

I'm afraid I wouldn't be very good company.

The two men were business partners.

Whether the characters in a book are real or completely imaginary, they can become your friends and companions.