On January 20, 2017, an SM Entertainment representative revealed that Red Velvet is planning on making a comeback in February.

Billboard described the title track "Rookie" as a "pop-funk single that draws on Red Velvet's unconventional musicality to combine low-key sing-speaking and synth beats before blasting into the saccharine hook of a chorus".

The song was composed by Jamil 'Digi' Chammas, Leven Kali, Sara Forsberg, Karl Powell, Harrison Johnson, Russell Steedle, MZMC, Otha 'Vakseen' Davis III and Tay Jasper with lyrics by Jo Yoon-kyung and was produced by The Colleagues.

[6] "Little Little" is an R&B pop song written by Gifty Dankwah & Bruce Fielder and was penned by JQ, Jo Min-yang of Makeumine Works and Park Sung-hee of Jam Factory.

"Happily Ever After" is a pop dance track composed by Sebastian Lundberg, Fredrik Haggstam, Johan Gustafsson, Courtney Woolsey and Deez with lyrics by Song Carrot of Jam Factory.

"Talk to Me" is a mid-tempo song composed by Kervens Mazile, Annalise Morelli, Alina Smith & Mats Ymell and was penned by Lee Seu-ran of Jam Factory.

"Body Talk" is an R&B pop song written by Sebastian Lundberg, Fredrik Haggstam, Johan Gustafsson & Ylva.Dimberg with lyrics by Misfit & Jo Yoon-kyung.