An animal companion is a loyal animal ally granted by a class feature or other character option. It is controlled by the player whose character has the feature, but it is not a second player character.
An animal companion shares its master’s initiative and acts immediately after its master. Its granting feature defines what it does when it receives no direct command. For example, a Beastmaster’s companion acts independently under the control of the GM or the game’s AI, using only basic instinctive actions such as attacking the nearest enemy it can reach, defending or staying near its master, or retreating from danger.
The feature that grants the companion determines the action cost and which commands it can understand. When a companion acts independently, direct commands override its basic behavior. Training can teach it additional actions and tactics.
The companion uses an appropriate animal creature stat block selected with the GM. Its statistics, advancement, recovery from injury, and replacement rules are determined by the feature that grants it. A companion can gain training feats as it advances; those feats can teach it specialized behaviors, such as rescuing an ally or coordinating attacks with its master. Unless the granting feature says otherwise, a character can have only one animal companion at a time.