The new Final Fantasy 7 Remake Yep Red XIII is playable via save editing
The new Final Fantasy 7 Remake save file editor can make Red XIII a playable character, albeit with significant limitations.
The save editor is the job of Xeeynamo, and in fact it is the Kingdom Hearts save editor, supplemented with Final Fantasy 7 Remake options. The software allows you to change the decrypted save from the game, such as managing inventory, matter and levels of characters. The amazing thing that this allows you to do is that you choose who in your group, including Red XIII.
Red XIII is not a playable character, so editing it in your group means that there are serious limitations on what you can do with it. For example, you can see in the tweet above that he does not have a special motion assigned to the Triangle button. However, he has his own unique attack animations, as they were designed for him to use as a member of the AI guest group.
Earlier this year, Square Enix explained why Red XIII is not a playable character in Final Fantasy 7 Remake; the developers decided that since he will be present only in the last hours of the game, it makes no sense to have him as a full member of the party. The second game in the series, which will take place after the party leaves Midgar, is already under development, and I hope Red XIII will be fully playable in this game.
However, the biggest limitation is that he leaves the party when the game goes to the cutscene or when other members of the group leave. Since this is just a save editor, not a modification (modifying PS4 games is harder, if not harder), it is unlikely that Red XIII will ever be more than a temporary game character on the console. Nevertheless, this suggests that if there ever will be a version of the game for the PC, then the modders will be able to turn it into a more permanent teammate.