Companions are units from other players that you can take with you on certain quests. Your friends can use your companions and you can use your friend's companions.

How do I set a companion?

Tap on the "Friends" icon on the bottom right of the menu, then select "Companion Settings."

Is equipment for companions separate from equipment for other parties?

Equipment for companions is considered separate, as is with equipment set for Arena. However, companions share equipment and espers with other companions., similar to a party.

Once you use a friend's companion, all companions set by that friend will be unavailable for 3 hours.