since we dropping theories i gotta feeling its shinpei, considering the firefly on the hand, which i never noticed before kollil
Honestly it could be because of the home near the fireflies thing. Or it could also be bait and the couple did in fact live for a long time together. OR the sadder more likely truth it's SATOKO because SHIMPEI dies and ends up giving her his heart for transplant or something.
WHAT IF THE PLOT TWIST IS THAT SOMEONE ELSE IS READING THE LETTER IN THE FIRST CHAPTER BUT SATOKO AND SHIMPEI ARE BOTH DEAD!!!