This is a missable achievement, Let me break it down, After the first cut scene with 'Andre' when you actually meet the black-silouhette character is finished, You will resume control of Dysis during the daytime, Return left to the last puzzle was solved then you will notice a new character on the top, To be able to reach it you need to press up on the D-Pad to locate the character while doing that press start you will notice the word “Aurelio” below the right corner of the screen, Once you confirm that you saw it quit the game and press continue from the main menu, Then you will be able to solve the puzzle and reach the mysterious character and the achievement well be unlocked.
Hope this helps!
Special thanks to the developers and WhiteThorn Digital for providing me this guide. 😊
You can also check this perfect achievement guide by Cheevo Guides for this game 👇