The single, and a remix by Russell McNamara (under the alias PlanitB), was used as the official Japanese theme song for the 2002 action role-playing video game Kingdom Hearts, and appeared on its original soundtrack respectively.

It was certified double platinum by the Recording Industry Association of Japan (RIAJ) for physical shipments of half a million units.

An accompanying music video was shot by her then-husband, Kazuaki Kiriya; it features Utada washing dishes and drinking water.

To promote the international formats of Kingdom Hearts, Utada re-recorded an English language version entitled "Simple & Clean".

[2] According to Nomura, he only had Utada in mind to create the theme song for the video game, so he had contacted them to collaborate; as a result, she accepted his offer.

[3] In a brief interview with IGN, Nomura further stated; "Her music has moved millions of fans, and I was absolutely thrilled when she agreed to contribute to this project.

[4] The song's instrumentation consists of keyboards and programming handled by Kawano Kei, synthesizers from Tsunemi Kazuhide, and an acoustic guitar from Akiyama Hironori.

[9] Square Enix Music's Neo Locke described the song's composition and melody in an extended review: "The acoustic guitar combined with the synth in the background creates a pleasant and gentle harmony that helps bring out Utada's voice.

"[10] A reviewer from OngakuDB.com noted the acoustic guitar as one of the composition's key elements, and described its sound as "melancholy" and a big "impact".

editor Hiromi Yonemoto noted that the acoustic instrumentation was an "unusual" change in Utada's normal pop musical style.

[13] According to Kano, the editor in chief of Rockin'On Japan, he stated that the lyrical content discusses themes of mystery and daily life actions; he furthered believed that the song's lyrics is an open interpretation, due to its lack of major characteristics and identified philosophy and religion as examples.

Neo Locke from Square Enix Music was positive in his review, saying "'Hikari' has always impressed me for having a very recognizable and easy to manipulate melody despite the fact that the vocals are the only melodic line in the piece — partially due once again to Hikaru Utada's strong and versatile voice."

editor Hiromi Yonemoto believed that "Hikari" demonstrated some of Utada's best vocals to date, and labeled them and the song's melody as "synergistic".

[15] Despite Daniel Kalabakov from Soundtrack Central disliking pop songs, he complimented Utada's singing and the track's instrumentation.

According to Naomi Gingfold, writing for The Global Post, she commended the departure of Utada's general "beautiful and intricate music videos", stating "The camera did not move once.

"[30] Daniel Montesinos-Donaghy from Noisey Vice noted and complimented Utada's abilities in adapting to different roles through her music videos, specifically highlighting the "mundane" activity of washing dishes.

[38] To promote the international formats of Kingdom Hearts, Utada recorded an English version of "Hikari", named "Simple & Clean".

[41] Utada explained the song process in a detailed interview with Jetanny Magazine: "... [T]hat was so hard, it's just, and it felt strained, and as a result, I'm happy that I worked hard to do those, because those English versions are really good and "Simple and Clean," I think, is a really good song, and people—most of the people that know me here, they know me for that—but it's not ideal for me as a writer, to—because, actually, I changed the melodies for "Simple and Clean" and "Hikari," because when you change the language you're singing in, the same melodies don't work—and as a writer, it's just very frustrating to have, like—I wrote these melodies for Japanese words, and to have to write in English for that, it's just not right.

And then, so, for this, uh, this contract with Island Def Jam, in the beginning I separated it to this English language album, and I don't do Japanese translations.

[43] The original edit and PlanitB remix of "Simple & Clean" first appeared on Utada's single "Colors" as a B-side, which was released on January 29, 2003.

[40] Near the end of 2003, "Simple & Clean" was released as an A-side 12-inch single with "Colors" in Japan; it included the original and PlanitB remix.

Benjamin Turner from GameSpy was impressed by the translation of "Hikari" into English, and felt Utada's vocals were a good addition to the opening and ending segments of the game.

[46] Michael Pascua from BlogCritics.org was generally positive, stating in a detail review; "Utada made a smart decision with the physical release of the CD: she included the songs "Simple and Clean" and "Sanctuary" from the Kingdom Hearts series.

Its first performance was at a special event that celebrated Utada's 20th birthday in Japan on January 19, 2003; she sang "Simple & Clean" as the encore track.

When the single was released and promoted through Kingdom Hearts, "Hikari" and "Simple & Clean" were widely considered a "hot topic" around the world of music, as described by a staff member at OngakuDB.com.

[57][58] Due to the success of the songs, Utada was invited to record another track for the original video game's sequel, Kingdom Hearts II (2005).

[62] Alex Hanavan from The Young Folks listed the orchestral version of "Hikari", which also appeared during the credits section of Kingdom Hearts, at number two on the "Top Ten Video Game Theme Songs".

He stated his reason through his extended review; "Kingdom Hearts has several 'theme songs' but the orchestrated version of 'Hikari' takes the cake with all the makings of a grand adventure.

"[63] GameFAQs's editor Pierce Sparrow listed both "Simple & Clean" and "Sanctuary" at number two on the "Top Ten Lyrical Songs for a Video Game".

In 2008, the Guinness World Records listed the track as the best-selling video game single in Japan, and was included on the 2008 Gamer's Edition book; this is Utada's first, and current, induction into this.