You can download the patch from the official Capcom website: http://www.capcom.com/support/sfxtpc_patch.html
Capcom released an official patch for Street Fighter X Tekken on PC, which includes various bug fixes, balance changes, and... character unlocks ! street fighter x tekken pc patch 108 character unlock link
** GameFAQs and Steam Community:** You can also check the GameFAQs page for Street Fighter X Tekken on PC: https://gamefaqs.gamespot.com/pc/2420-street-fighter-x-tekken/ You can download the patch from the official
Some Reddit users shared a link to a thread with a : https://www.reddit.com/r/StreetFighterXTekken/comments/1jyy13/character_unlock_tool/ which includes various bug fixes
Or, visit the Steam Community page for SFxT: https://steamcommunity.com/app/1210
You're looking for a PC patch to unlock all 108 characters in Street Fighter X Tekken!