I knew about this song and also about the TV comedy drama "Trick" by reputation, but didn't realize that the two were connected. Chihiro Onitsuka's（鬼束ちひろ）2nd single from August 2000, "Gekkou" (Moonlight) was indeed the theme song for the original Friday night series starring Yukie Nakama and Hiroshi Abe（仲間由紀恵・阿部寛）as a down-and-out magician and arrogant physicist respectively joining forces as debunkers.
However, I knew about "Gekkou" just on its own as this sad if soothing ballad sung by this angelic voice that I had never heard before from anyone. Onitsuka wrote and composed the song about someone who might either truly be an angel or just a regular human being despairing over the state of planet Earth. In any case, the original music video also had a couple of religious references starting with the Christian cross and then Onitsuka appearing like a backlit angel for the most part. There is something quite gospel-like about "Gekkou" and how it's delivered.
Both "Trick" and "Gekkou" became hits. Apparently, the latter was never meant to become a single but with the hit status of the former, the song was quickly pushed into release. And although it didn't quite break the Top 10, it did hit No. 11 and went Double Platinum before becoming the 71st-ranked song for 2000. The ballad is also available on her debut album, "Insomnia" from March 2001 which hit No. 1 and was the 16th-ranked album of the year.
As for Onitsuka, she is originally from Miyazaki Prefecture, and her name came about from the lyrics of a Masashi Sada（さだまさし）song, "Saijiki"（歳時記...Diary）back in the late 1970s. Her interest in music came about when she was in Grade 6 of elementary school when she really got into the music of Carl Smokey Ishii from Kome Kome Club（米米クラブ）and then later, Koshi Inaba（稲葉浩志）of B'z.