It's not the wallet, it's the ring on top that's important!
What does it mean, I kinda forgot
i think that's a diary from the seme's past and i feel like he knew and remember uke all along
Yes it is a diary! The ring is important because it's the one yoonsung proposes to haekyung with.
It comes up a lot in the story, like seme asking uke not to sell it off, uke not being able to part with it even when the seme wants to throw it out, and even..
******SPOILER********
the seme bought it for the uke after discovering he's a man! And I bet it will come up a few more times too.
What was in that wallet???