A Letter From Mars 2003 on netflix

Little Seung-jae stays up all night and writes letters to So-hee, pretending to be her father in Mars. Leaving Seung-jae’s innocent heart behind, So-hee leaves for a big city. Both grown up to be adults, Seung-jae is a postman who writes letters to So-hee’s grandma – the letters that never came from So-hee. So-hee’s heart hardens after hardships in life and she comes back to her hometown and Seung-jae’s heart swells up again at seeing her again…