24°C

On January 15, 2019, Lee released the digital single "XI" with Code Kunst; the track was sung live for the first time on MBC Dreaming Radio.^[5][6] Lee received her first music show win for single "No One" through Mnet's M Countdown on June 6.^[7]

At five tracks, 24°C is the shortest material in Lee Hi's catalogue. The album explores the dark side of romantic relationships, the loneliness and the longing, the frustrating relationships, and the break-ups.^{[8][unreliable source?]} Musically, 24°C combines these mostly darker themes with generally upbeat musical genres.

The opening track of the album, "No One" featuring B.I of iKon, as a reggaeton-infused dance track with a "groovy" tune full of vibrant percussion, smooth melodies, and quirky synths. It expresses the longing of a lonely person waiting for a lover to arrive with a knock on her door.^[9] Lee Hi's collaboration with G.Soul, "No Way", is an "unashamedly laid back track" with a piano and a low bass. "Love Is Over" is "a gospel, soul-infused ballad" with a jazzy swing rhythm. It functions very much as a typical break-up tune, reflecting on the struggle to move on the relationship. "1,2" featuring Choi Hyunsuk of Treasure, is a "short but sweet" track. "20Min" is the only self-written song by Lee on the album. The "guitar-led ballad" details her anger and annoyance with a certain someone over gentle guitar strums and the sound of water-drops.^{[10][unreliable source?]}

Promotion for the EP began following a coming soon poster released by YG Entertainment on May 20, 2019.^[11] On May 22, the date and title of the EP were unveiled, with pre-order details following on the same day. On May 25, a concept teaser of the lead single "No One" premiered, and soon after, the track list was revealed on May 27.^[2][12][13] Lee's first music show appearance was on MBC's Show! Music Core on June 1.^[14] On June 5, Lee had a live stage broadcast on Naver Vlive with Vibe, with 40 fans chosen to attend the showcase.^[15] On June 11, Lee appeared on JTBC's Idol Room.^[16]