Written by Lee Thor (Lalala Studio) while production handled by G'harah "PK" Degeddingseze, Patricia Battani, and Steve Octave, "Illusion" is an electronic "high-energy" hip hop, synth-pop and dance song with bass-heavy chorus, and elements indicative of hyperpop.

[1] Alongside the announcement, Aespa surprise released "Illusion" as a pre-release track from the EP, accompanied by "futuristic-looking" lyric video on the same day.

[3] "Illusion" was written by Lee Thor (Lalala Studio) with composition and arrangement handled by G'harah "PK" Degeddingseze, Patricia “Tricia” Battani, and Steve Octave.

Crystal Bell of Nylon described the song as "synthpop siren's call with a deliciously dirty bassline", and that it "highlights each member's vocal charms".

[10] Writing for Consequence of Sound, Mary Siroky named the song as "a high-energy 'sneak peek' track" that offers "a vibrant beat and an earworm chorus".

[13] Rolling Stone's Kristine Kwak stated the song is "a loser to Aespa’s traditional futuristic synth-heavy sound" and called it "a fan favorite and rightfully so".

[14] Writing for Beats Per Minute, Chase McMullen labelled it as the "sort of song that has the power to get you dancing in the middle of a bar, not caring the least about an onlooker as you weave to its every move".