If you still don't know how to do it here is a video from DoomnightK, go to 1:45 to see:

To make sure it will work you can start spamming your character key before you press restart to make sure you load in as them.

It's important that you start doing it when the screen goes black because if you start when you already see the loading screen it will be too late and it won't work. When you load into Story Mode, pause then restart, and when the screen goes black immediately start spamming the key assigned for the character you want to spawn in. If neither DLC folder works just place it in the Config folder of any other DLC folder it doesn't really matter which one. If that doesn't work try placing BmInput.ini in: Batman Arkham Knight > DLC > 356474 > Content > BmGame > Config rar file then place BmInput.ini file in: Batman Arkham Knight > DLC > 313100 > Content > BmGame > Config If the character comes from DLC like Harley Quinn or Red Hood you need their DLC to load in as them. So if you wanted to do a boss fight during the story like the Arkham Knight fight as a different character like Red Hood you can use this mod. This mod gives you the ability to spawn as different Arkham Knight characters from your last saved checkpoint.

Batman arkham knight free roam mods