A Companion is a character who accompanies the playable character -- the Warden in Dragon Age: Origins, the Warden-Commander in Dragon Age: Origins - Awakening, Hawke in Dragon Age II, Legends Hero in Dragon Age Legends and the Inquisitor in Dragon Age: Inquisition.
Companions have their own opinions and objectives and may ask for the player's help on quests of their own. Only three companions at a time can travel with the player, but a full party does not prevent the player from recruiting further companions. Once recruited, any companions not chosen to join the party will wait at the camp.
Companions who are not actively involved in the party will still receive experience and level up with the main character. Using the "auto level-up" option, they will level up automatically and choose default stats improvements depending on their class. Without auto level-up, the player levels them up the next time they are selected as an active party member, or the next time they are in camp. Some companions in some installments can also be romanced by the playable character.