Here is a short guide for the secret achievement.
Note: This guide was created by KiruChan and there are two clues with a long blank in order to avoid spoiler.
How to get Secret Achievement
The second clue its
If you still dont find the achievement you only have to put (in the main menu) the konami code that its:
With a Xbox Controller (Recomended):
UP, UP, DOWN, DOWN, LEFT, RIGHT, LEFT, RIGHT, B, A.
With a PlayStation Controller:
UP, UP, DOWN, DOWN, LEFT, RIGHT, LEFT, RIGHT, O, X.
If u dont recieve the achievement maybe u are doing it too slowly.
Thanks to KiruChan for his great guide, all credit to his effort. you can also read the original guide from Steam Community. enjoy the game.