I'll be honest, I didn't beat the piano puzzle, even after help I couldn't process what pieces to do in order. But I do love the atmosphere and the hints are good, it's just that there isn't any clear way to tell what you're missing.
(I am also biased towards this because my original plan has a bit of this vibe before I realised I overscoped and started over)
The puzzle isn't that difficult, you just need some observation. There is actually only one puzzle that you need to solve and it's the diary. Just do all the task what diary says and that's it. I agree about the piano thing as it's pretty hard to memorise the tune. So i can help you with the tune, mark 1-9 all white keys from left to right, the currect tune is 3276. You can try again if you want. Anyway thankyou for playing. :)
