Guess Who was subject to generally mixed reviews from music critics, who complimented the record's cohesiveness; however, some found the material controlling the vitality of the group.

To promote the EP, the group performed on several South Korean music show programs, such as M Countdown and Inkigayo.

The album features six songs which incorporate elements from country strings to lo-fi rhythms, and showcase the band's vocal skills.

[13][8] "In the Morning" was co-written and co-produced by JYP Entertainment founder Park Jin-young, who had previously worked on two of the group's hit singles, "Icy" and "Not Shy".

[8] Other producers featured on the record include hitmakers Earattack, who worked with Got7, and frequent Kang Daniel collaborator JQ.

[8] During a press event for the EP, the members shared that the title of Guess Who refers to how they show different emotions and personas in the songs while exploring their musicality.

[15] The second song, "Sorry Not Sorry" is a "playful" and "laidback" track with a bouncy rock-inflected production that "highlights the title's devil-may-care attitude".

[9][14][16] The song combines the five members' "sweet" voices with ordinary instruments such as an acoustic guitar and analog drum beats.

[21] On May 1, Itzy appeared on the South Korean TV show Knowing Bros.[22] "In the Morning" was released as the lead single from Guess Who on April 30, 2021.

[23] Itzy promoted the song with televised live performances on various South Korean music programs including M Countdown, Show!

[24] Sofiana Ramli of NME awarded the extended play 4 out of 5 stars, describing it as "by far Itzy's most cohesive project in terms of sound".

[16] The reviewer commended the hip hop musical direction on this record as the girl group "seem to have successfully found a brand of maximalist pop that works for them.

"[16] In a mixed review, Joshua Minsoo Kim of Pitchfork found that, although their debut "established the quintet as a massive new force in the industry", their latest release, Guess Who, was a disappointment, commenting that with the EP, Itzy "became more uncomfortable being themselves in the process".

[1] IZM's Dayeol Jeong felt the album "which must passively follow a set direction, controls the vitality of the five girls.

[28] The EP also entered at number 5 on the Gaon Album Chart for the month of April 2021 with less than a day of tracking and selling 232,570 copies.