Unlike in Final Fantasy 7, Red XIII is not a playable character in Final Fantasy 7 Remake. Instead, Red helps the party autonomously as a "guest" member, somewhat similar to that of a FF7 Summon — only Red lasts forever and players can't tell him what to do. Producer Yoshinori Kitase said that the development team decided to go this route because it didn't feel like the game left enough time for players to become as familiar with Red as it did the other playable cast members.
That said, Red XIII still has his own uniquely animated fighting style and abilities, as they were necessary for him to function as a party member. And so, with a little save editing, Twitter user MP1stLara revealed that it actually is possible for players to take control of Red as a playable character. But when they do, the character will suffer from some strict restrictions.
While using Red, players won't be able to build ATB gauges, which allow them to use their party's more advanced techniques and Materia-set spells. This means Red XIII won't be able to pull off his usual moves like Cure and Stardust Ray. Red also doesn't have a unique ability anchored to the triangle button like the other cast members, so all that's left to him is his basic combo.
Players will also have to deal with the fact that Red leaves the party the moment another character does or if the player enters a cutscene. So users will have to jump through hoops just to use Red consistently. These shortcomings exist because the ability for players to take control of Red comes from a save editor rather than a modification. And it's almost nearly impossible to mod a PlayStation 4 title, so players will more than likely be stuck with these limitations when using Red XIII in FF7 Remake.
Modifications on the PC are easy by comparison, and Square Enix did reveal that a PC version of FF7 Remake does exist. Square also stated that FF7 Remake would only be a PlayStation exclusive for a year, so there's a good chance then that the title will make its way over to PC. If it does, modders should be able to turn Red XIII into a complete character or at least something close to it.
In the meantime, the save editor that allows players to use Red comes from Xeeynamo, who typically uses it for Kingdom Hearts, but recently updated it to work with Final Fantasy 7 Remake as well.
Final Fantasy 7 Remake is available now for the PlayStation 4.