Written and produced by Peter Rycroft (Lostboy), Lauren Aquilina, Ryan S. Jhun and Alawn with additional songwriting credits by Seo Ji-eum, "Eleven" is a dance-pop song that lyrically expresses the members' romantic feelings.

[5] "Eleven" was written by Seo Ji-eum along with Peter Rycroft (Lostboy), Lauren Aquilina, and Ryan S. Jhun, whilst production and arrangement was handled by the latter trio and Alawn, respectively.

[6] Musically, "Eleven" is a dance-pop song that utilizes oriental instrumentations in the introduction with repeated percussion rhythms and marimba melodies, while its lyrical content expresses "the heart of a girl who is in love".

[7][8] Teen Vogue wrote that "the song, which features a light but snappy percussive instrumental, bucks K-pop trends by slowing its pre-chorus down, ramping up anticipation before detonating its catchy chorus.

Writing for webzine IZM, Jung Jeong-min complimented the track's production, particularly in the slow pre-chorus, and wrote that "overall, it is a debut single that exceeds expectations".

[7] All three music critics from webzine Idology gave the song a favorable review, where they highlighting its unique composition and the group's performance.

[30][31] Prior to the single album's release, on December 1, 2021, Ive held a debut showcase to introduce Eleven along with its title track.

[33][34][35] On the December 8 broadcast of MBC M's Show Champion, Ive received their first music program award for "Eleven", doing so a week after debut.