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Where can I find the save file? I found only a "PlayerPrefs.txt" for this game.
This question / problem has been solved by wonderfulillusionimage
Sorry for the necro, but for anyone else who is wondering, PC Gaming Wiki has you covered.
%USERPROFILE%\AppData\LocalLow\Zeboyd Games\Cosmic Star Heroine\
That's the location where I found the "PlayerPrefs.txt". It seems that this is the save file.