
It is the ML reading the letters that MC left.
“ A very old letter:
" Women are property and are exchanged like objects as gifts, don't get married "
Read by an old man, crying, wearing a simple kimono by the lake ML had shown to MC with fireflies as if read as many times they could have read it, tattered in an envelope.
He says to himself:
" And yet I did it myself, I fell in love with someone I shouldn't have"
Lonely and alone”

It is the ML reading the letters that MC left.
“ A very old letter:
" Women are property and are exchanged like objects as gifts, don't get married "
Read by an old man, crying, wearing a simple kimono by the lake ML had shown to MC with fireflies as if read as many times they could have read it, tattered in an envelope.
He says to himself:
" And yet I did it myself, I fell in love with someone I shouldn't have"
Lonely and alone”

It is the ML reading the letters that MC left.
“ A very old letter:
" Women are property and are exchanged like objects as gifts, don't get married "
Read by an old man, crying, wearing a simple kimono by the lake ML had shown to MC with fireflies as if read as many times they could have read it, tattered in an envelope.
He says to himself:
" And yet I did it myself, I fell in love with someone I shouldn't have"
Lonely and alone”

OMG I am so sorry I didn’t see it was yours, I shouldn’t have used it without giving you credit .. My apologies.
And yes, that proves it, atleast so I understood it, the fact that the old man is the ML. And if my memory serves me right, there was/is? a web novel and the ending was that similar of what the old man says: he fell in love with whom he shouldn’t have (MC) and consequently suffers the consequences. In the novel as far as I know, they made it out but then a series of unfortunate events happened and them came the time skip and that old man says that. But there could also be a possibility that this won’t happen since authors don’t always follow what the web novel says or that my memory is wrong and there is no web novel

No I didn't mean it like that, use my comment where ever u want, I wasn't asking for credit as well.
I thought u took my comment as spoiler so i was feeling bad ╥﹏╥
And i read a theory that it's not ml but sml since it doesn't make sense as their is no one ml or fl has who would write them a letter like that nor does it make sense fl wrote it to ml.
But still even if it's not ml it's still so freaking sad, my poor boy has suffered alot as well, give our smls a happy ending too they deserve it (/TДT)/

At this point...it was shown that Mc is getting thinner and her skeleton are showing on her chest already She have heart disease and she said she want mL to be happy even when she's gone so it's likely mc die...since she's already a skeleton in manga And it look like ML was the old man who is still left in the island with fireflies? ML is just straight up having series of tragedies
So I reread the first chapter and have my theory..that person who holding the person look more like a grandpa than grandma
Maybe it's ML reading mc letters she left?.. it's just my theory tho