with U is a crossover track originating from MARVELOUS Games' rhythm game WACCA Lily, where it served as the theme song for the Lily version update. Jointly composed and arranged by t+pazolite (Tomoyuki Hamada) and Massive New Krew, the song is performed by Lily, the navigator character introduced in WACCA Lily, voiced by Shiki Aoki. The track was added to maimai でらっくす BUDDiES on November 10, 2023, following its earlier debut in SEGA's ONGEKI (bright MEMORY) in July 2023. At 155 BPM, with U delivers an upbeat, pop-infused energy characteristic of both t+pazolite and Massive New Krew's electronic-oriented production styles. The lyrics, sung from Lily's perspective, express unwavering devotion and a promise to stay by someone's side, with recurring phrases like "I'm with U" and "あなたの側にいるよ" (I'll be by your side). The jacket art was contributed by LAM, a prominent illustrator whose work appears across multiple WACCA and maimai titles. In maimai, the song's Expert chart was designed by サファ太 (Safata) at level 12, while the Master chart was handled by Luxizhel at level 13 (constant 13.4). The Master arrangement is notable for its unusually high count of 162 Hold notes and 86 Slide notes out of 797 total, creating a chart that emphasizes sustained sweeping gestures, a signature trait of Luxizhel's design approach.