Monster is the debut extended play by Red Velvet – Irene & Seulgi, the first sub-unit of South Korean girl group Red Velvet. It was released on July 6, 2020, by SM Entertainment, with Dreamus serving as the South Korea distributor. The EP contains six tracks, including the title track of the same name and a follow-up single "Naughty". Background and release On April 21, 2020, SM Entertainment confirmed that Irene and Seulgi would form Red Velvet's first sub-unit and that the duo was in preparation for their first major release in around June 2020. The release of the mini album was postponed from June 15 to July 6, 2020, on the grounds that it needed additional production on the album, in order to achieve a higher quality of music. The physical release contains two different versions with a CD, a photo booklet, a lyric book and collectibles. Commercial performance According to Hanteo, Monster sold over 80,000 copies on its first day of availability, beating Red Velvet's first day and first week album sales records of the group's sixth extended play The ReVe Festival: Day 1, which set the record at 45,080 copies for its first day, and 71,431 copies for its first week. It also became the best-selling girl group sub-unit album of all time, surpassing Twinkle (2012) by Girls' Generation-TTS. The EP reached 100,000 copies sold on Hanteo in under 3 days, making it the first Red Velvet album to do so. The EP debuted at number five on the Billboard World Album Chart, becoming the duo's top-five entry. Track listing Notes: "Naughty" is only available in the EP as a CD-only track. It was omitted from the digital release of the EP until July 20, 2020, when it was released on Korean music sites as part of the EP and as a standalone digital single in international music sites. Credits and personnel Credits adapted from the liner notes of the EP. Musicians Red Velvet – Irene & Seulgi Irenevocals , background vocals Seulgivocals , background vocals KenzieKorean lyrics JQ (makeumine works)Korean lyrics Lee Yeon-ji (makeumine works)Korean lyrics Kim Yeon-seoKorean lyrics , composition , background vocals Lee Seu-ranKorean lyrics Park Green (153/Joombas)Korean lyrics Yaakov "Yash" Gruzmancomposition , arrangement , drums , keyboards & synths Delaney Janecomposition , background vocals Jenson Vaughncomposition Yoo Young-jincomposition , arrangement Moonshinecomposition , arrangement Louise Frick Sveencomposition , background vocals Charite Vikencomposition , background