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SPOILERS AND CHEATS FOLLOW
YOU HAVE BEEN WARNED!!
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They should unlock when you choose a ‘five-heart’ character at the end.
I will tell you a secret way to get around this! (It’s mentioned in one of the character’s unlocks, so it’s not super-secret.) But first… either you’ve stumbled upon an error in the game nobody has found or mentioned yet -or- the playthrough when you chose Max, you didn’t have five hearts -or- what I’d say is the most possible given error reports: windows didn’t feel like allowing the game to save the data 🙁
Anyway, the secret:
(((navigate your way into the game’s installation directory, default is “C:\Program Files\Fupoco\Common Grounds”, open the ‘data’ folder, open the file “0_6a.txt”, and type in a new line somewhere before the ‘/goto’ line at the end, this: “/unlock max4” and in another new line: “/unlock max5” — save the file and close it, then start a new game and check Max’s bonus unlocks! wink wink~)))