Responses
I think Jin was the girl.
Well it as as Kai said, it was buried,I guess at the root of the tree that we see there: http://www.mangago.zone/read-manga/koujitsuei_no_tobira/bt/171692/Vol1_Ch65/5/it was in California where the three of them lived together an the girl is that little girl that gave Kai the brother's last belongings there: http://www.mangago.zone/read-manga/koujitsuei_no_tobira/bt/171691/Vol1_Ch6/22/
Wait. So I' assuming that the "journal" Kai was talking about was in the girl's memory? The girl at the end? Anyways, this was such a good read, I loved the plot twists.