The save file for Street Fighter X Tekken on PC is usually located in:
Modifying save files can potentially harm your game or account. Proceed with caution and at your own risk. Street Fighter X Tekken All Characters Unlocked Pc Save File
C:\Users\<YourUsername>\AppData\Roaming\Capcom\SFxT\Savedata The save file for Street Fighter X Tekken
To unlock all characters, you'll need to edit the sfxt_save.dat file. You can use a hex editor like HxD or a save editor like Savegame Editor to modify the file. Street Fighter X Tekken All Characters Unlocked Pc Save File