While there is no "official" patch from Capcom to unlock DLC characters for free in the PC version of Street Fighter X Tekken (v1.08)
Locate the File: Navigate to your game installation folder, typically found at: ...\Street Fighter X Tekken\resource\CMN\boot\game
The "1.08" patch is a specific version often associated with balance changes and community efforts to keep the game playable after Capcom removed Games for Windows Live (GFWL). 10th Anniversary Edition : A comprehensive community mod available on the Steam Community street fighter x tekken pc patch 108 character unlock link
SFxT_v1.08_Character_Unlock.rar).Download Link for Patch 1.08
What the patch does:
Official Purchase: While the DLC characters were originally paid content, the game's removal from many digital storefronts has made community patches the primary way for many players to access the full roster.
Do not download from "cheat" websites that ask for surveys. The legitimate file should be roughly 15MB to 30MB (it is a patch script, not a multi-gigabyte reinstall). The full mod pack (with costumes/huds) is roughly 500MB. While there is no "official" patch from Capcom
Change the value 'dlc_b' to 'default' for every DLC character you wish to unlock.