Throughout the series, the story of First Love parallels to many of Hikaru Utada's song lyrics and titles. "Distance" and "Letters" (both released around 2001) parallel with their lyrics and titles to when Yae gets into a car accident, losing her memories, and Yae and Harumichi end up separating. Harumichi attempts to give Yae letters, but her mom hides them from him, thus mirroring the two mentioned songs.
Harumichi's sister is deaf and can only communicate with sign language without using her voice. Despite this, Harumichi keeps saying "Shut up" to her multiple times in all timelines throughout the series.
Towa Araki and Takeru Satoh worked together on the movie Rurouni Kenshin: Final Chapter Part II in 2021.
The scene where Yae and Harumichi kiss in the snow in the very first episode parallels to when they kiss in the snow as adults in the final episode.
You see the young couple walking by the seashore and in the background you see wind turbines out in the sea. The time is 1998 but wind turbines were not installed in Japan until 2013.