Tainted characters are stored via a separate bitmask. If you only unlock Tainted Cain but not Tainted Isaac, the game may freeze when scrolling through the character selection screen. Always use the "Unlock All Tainted" button if you unlock one.
Not all editors work with Repentance. The DLC fundamentally changed the save file structure. Here are the three most reliable tools. The Binding Of Isaac Repentance Save Editor
Warning: Manual editing without knowing offsets/formats is likely to corrupt save. Tainted characters are stored via a separate bitmask
Status: The Gold Standard. Platform: Windows / Linux (Wine) / Mac (Mono). Why it wins: It is the only editor that fully supports all Repentance content, including the 34 Tainted characters, the "Beast" and "Mother" completion marks, and the "Death Certificate" unlock logic. Not all editors work with Repentance
Interface: A visual grid of character post-it notes. You simply click the icons to mark them as completed (Normal/Hard, Boss Rush/Hush/Mother/Beast/Delirium).
How to use: