https://www.gamespot.com/articles/final-fantasy-15-december-update-out-now-adds-char/1100-6455598/

A new update for Final Fantasy XV is now available on PS4 and Xbox One. Square Enix has rolled out the game's December patch ahead of its next DLC expansion, Episode Ignis, and it adds a notable new feature to the open-world RPG: character swapping.

With update 1.19, players will be able to switch between Noctis and his companions freely while in a battle. Previously, players were limited to playing through the game solely as Noctis, so this marks the first opportunity they'll have to control Prompto, Gladio, and Ignis during the main single-player quest. However, you'll need to unlock the ability to swap characters in the Ascension Grid before you're able to make use of it. You can see the new feature in action in the video below.