Last Letter (2020; ラストレター, Rasuto Retā)
At her school reunion, Yuri gets confused with her sister Misaki, which has shortly before died, and she doesn't bring herself to tell the others. A fellow student tells Yuri that he was in love with her, which leads her to write him letters, posing as her sister, from which on, we get the story of their school days, and the circumstances that lead to Misaki's death.
Liked it a lot. It's a »human« story, messy and unnecessarily complicated, as is a life.