The track was written by Rosé, along with Amy Allen and the song's producers Bruno Mars, D'Mile, Carter Lang, Dylan Wiggins, and Omer Fedi.

The song ultimately achieved moderate success, peaking at number 29 on the Billboard Global 200 and securing top-five positions in Hong Kong, Malaysia, Singapore and Taiwan.

"Number One Girl" was written by Rosé, Amy Allen, Bruno Mars, Dernst Emile II, Carter Lang, Dylan Wiggins, and Omer Fedi; it was produced by the latter five.

[7] It is a sentimental, pop-punk song,[8] featuring a piano ballad that lyrically delves into themes of heartbreak, unrequited love, longing, insecurity and vulnerability over a soft rock instrumental.

[9] Gabriel Saulog of Billboard Philippines labelled it as a "devastating opening track" to her album, opining that the powerful ballad highlighted Rosé's "golden voice" and heart-wrenching lyrics with its "embrace [of] piano-laden melodies and crashing drums".

[14] The Wall Street Journal's Mark Richardson described the song to be a "theatrical, swooning ballad", which he remarked was both a "statement of purpose" and a "bold choice to open a pop album".

[16] In his review of the album, Jeff Benjamin of Billboard ranked the song at number 5, praising Rosé's emotionally charged vocals and comparing its impact to Olivia Rodrigo’s "Drivers License".

[8] Recorded in the style of a retro camcorder, it depicts Rosé wandering at night around the streets of Seoul, a skate park, the top of a lookout, and the Jamsu Bridge, while she sings to an unnamed lover.