Hmmm...
It's a
beautiful Celtic-style soundtrack, sung in Japanese, and created by a French lady called Cécile Corbel and her band, who (I learned) also sang the English, Japanese, and French versions of the song/soundtrack (very, VERY cool).
This song style fits well with Kajiura's oft-used 3/4 waltz style and sometimes Celtic melodic and instrumental sensibilities, so an appropriate choice for Kalafina!
https://open.spotify.com/track/26IEfjU8v6z8dBeyXxQ1Lh?si=7ca4fe1952784329 - link to “Arrietty’s Song” from the artist.
https://open.spotify.com/track/4F22XJWhfi1BHI0fOA9td6?si=b4aa2b4de3974b7c - link to “Arrietty’s Song - Instrumental Version”
https://open.spotify.com/album/0ldESLoQyyP80ynVU2gych?si=tOAGI5gDQYywS5l9KS4O9Q - link to the entire Arrietty soundtrack.
From her Spotify bio (where "Arrietty's Song" has over 16 million listens): "This artist with a captivating universe was born in 1980 in Finistére, Brittany. She forges a unique and inspired path as a composer, author, Celtic harpist, and singer." ... Her work on the movie of the year in 2010, earned her the title of Best Original Soundtrack of the Year at the Japan Golden Disc Awards, as well as two gold records."
So, an award-winning composer and a beloved song. This will definitely be eagerly awaited by fans, and I await a listen as well.
For Kalafina,
the chorus is the real work, where the harmony will shine. For me, this is a song to be led by either Hikaru first on verse one, then Keiko on verse 2, and Wakana just joins for the chorus harmony. Hikaru actually has/had the right Elf-like voice for it (think "Hikaru no Senritsu"), but she can't sing gently like she used to be able to so it will be more breathy... I do think she can do it since the verse melody doesn't have high notes per se... The song is also acoustic so a good choice not requiring belted vocals and the voice can shine more naturally.
(P.S.:I can also see Haruka Shimotsuki covering this song since she's quite Celtic-influenced in her music and their voices are SO SIMILAR, in terms of color, forward placement and range!)