Hello Minna-san! I'm looking for a manga (though I don't really remember much from it).. and I'm not sure if it was the guy or the girl who died (but I think it's the girl) and it might just be a one shot or from a manga with multiple one shots..

from what I remember.. the "girl" was not aware that she is dead, she died from falling off a river during a typhoon because she was trying to get a hairpin? that was given to her by her boyfriend. Since she drowned there was always a puddle of water on the ground and until the guy became an adult he still remembers the girl.

Thank you very much.