Omg noooo hope it's not sad ending plz I wouldn't be able to take it
but the holder of the letter (and their clothes including their broad shoulders) looks like a man tho? i think it's more suitable to say that the letter come from satoko who died from her illness instead of shinpei
I think it's actually from the FL, she probably died young from her illness or something else happened
Or you're right and Shinpei died while protecting fl
The letter says something like: I love you with my own free will. No matter how it ends up, I say this from my heart.
So assuming that letter came from Shinpei, so they didn't end up happily is what I think. She married another and he most probably died young.