Stuck after Bamboo

[Albacoide]

I am really stuck after finding the letter inside the Bamboo.

I've spoken to all characters in person or on the phone and no one has anything to say.

The only remaining puzzles I can think of are the colored grids behind the front desk and opening Yumi's frog. Also, I have Rentaro's card with a star but don't know what to do with it.

Any hints are appreciated!

Thanks!

[catsandbooks]

Did you "origami" the paper? Also, have you found the red piece for that puzzle?
The card will become useful once you solve the colored grids and use what you find there.

[Albacoide]

Yes, I did the Origami, which directed me towards the Bamboo... I opened it and found a piece of paper with a code of numbers.

I haven't found the red piece for the colored grids puzzle. Where can I get it?

[catsandbooks]

Oh, my bad...memory glitch! The numbers will come in handy later.

Go back to the cultural center at night and look around for that red piece.