The English-language version of the song, titled "Simple and Clean", was used in the English and Final Mix releases of Kingdom Hearts, Kingdom Hearts Birth by Sleep and Kingdom Hearts Re:coded. Although the two songs share a similar melody and background music, the meaning of the lyrics are changed significantly in the English version. "Simple and Clean" was also used in both Japanese and English-language releases of Kingdom Hearts: Chain of Memories. Utada stated that she couldn't get the same meaning she wanted to portray in "Simple and Clean" as in "Hikari", so changed the chorus melody to the one used in "Uso Mitai na I Love You", also on the Deep River album. Both "Simple and Clean" and its remix, "Simple and Clean -PLANITb Remix-" were later released as the B-side of her single COLORS.
"Hikari" was performed during Utada's two date concert series Wild Life in December 2010.
The music video (directed by Kazuaki Kiriya) for "Hikari" is a fairly simple one. Utada washes dishes, and enjoys a glass of water through its course. Utada, in an interview, explains how she enjoys washing dishes at home as well, which contributed to her making this music video.
"Actually we were to shoot the PV of 'Hikari' with him (Kazuaki Kiriya) but it didn't come true due to his scheduling conflicts at the last moment and that's why we requested Kiriya urgently to shoot that dish-washing video." 
(They were able to reach Kazuaki Kiriya eventually, but they had no time to shoot a complex video.)
The single proved to be very popular. It hit #1 on Oricon charts for a lengthy three weeks, selling 270,370 in its debut week. By August 2002, it had sold over 598,130 copies in Japan. In 2008, Guinness World Records listed it as the best-selling video game theme song in Japan.