Takahiro Sato, the film's co-producer, approached Amuro about providing theme songs to help her music gain international exposure.

Musically, it is a pop ballad with lyrics that intertwine with the film's themes and concepts, highlighting the journey from sadness to strength.

Despite modest physical sales, the song was certified gold by the Recording Industry Association of Japan (RIAJ) for exceeding 100,000 downloads.

In June 2015, Amuro released her twelfth and final studio album, Genic, which was a critical and commercial success in Japan.

[1][2] After promoting the album with her Livegenic Tour in Asia, Amuro resumed releasing singles, beginning with "Red Carpet" in December, then "Mint" in May 2016, and "Hero" in July.

However, knowing that the series was already well-known around the world, Sato decided to enlist Amuro to help international audiences recognise the "power of foreign artists" while also promoting her work.

[b] According to Japanese critic Kanako Hayawaka, the track highlighted Amuro's "painful" delivery, which was accompanied by a gentle piano riff, and featured a theme change from "sadness to strength".

[12] A snippet of "Dear Diary" first appeared in the film's trailer and was later leaked online in low quality on September 26, 2016.

By the first chorus, a distant shot of the ocean reveals glass bottles that house multiple homes, people, and animals.

As the final chorus begins, the bottle tops melt away, revealing a large rift of water beneath a newly risen sun and multicoloured clouds.

[23][24] It debuted at number three on the weekly Oricon Singles Chart, selling 45,000 units, her best first-week performance since "Go Round" / "Yeah-Oh!"