A companion was thought to be a character controlled by AI that each player could choose to use. It is, however, no longer in the game as Arenanet has decided to remove it completely.
Companions were planned to have customization similar to the Heroes of the original Guild Wars game series. Players would be allowed to bring a single companion with them, who would be considered an extension of the character and not take up a party slot. When a player chose not to bring a companion, their character would instead be buffed.