Step 1. Locate save at C:\Users\Potara\AppData\LocalLow\Arc System Works\GUILTY GEAR\savedata
There should be only one file, save.dat
Step 2. Put your save into https://www.saveeditonline.com/
Edit the value "characterCount" to be 13.
Step 3. Replace your old save.
When you get into the game, all 13 characters will be available.