異世界かるてっと (Isekai Quartet) is the opening theme of the 2019 anime crossover series Isekai Quartet, a comedic chibi-style crossover that brings together characters from four hit isekai series: Overlord, KonoSuba: God's Blessing on This Wonderful World!, Re:Zero -Starting Life in Another World-, and The Saga of Tanya the Evil. The song was composed, arranged, and written by Masayoshi Ooishi (大石昌良), a prolific singer-songwriter known for other anime theme songs such as "Kyouran Hey Kids!!" and "Himitsu no Koto". Each section of the song is sung in character by the four main voice actors—Satoshi Hino (Ainz), Jun Fukushima (Kazuma), Yusuke Kobayashi (Subaru), and Aoi Yuki (Tanya)—resulting in a call-and-response pop arrangement with a lively marching-band brass section and character-appropriate spoken interjections. Added to maimai during the maimai でらっくす PLUS era, Isekai Quartet was initially available through a limited-time area (ちほー) from January 23 to March 5, 2020, before becoming a standard unlock from March 6. The song was later removed from arcade cabinets on September 15, 2022, and is no longer officially playable in standard rotations. It also appeared in CHUNITHM PARADISE LOST as part of a MAP event. For maimai players, the song offers four DX charts. The MASTER chart, originally rated 12+ and later adjusted to 12, was designed by はっぴー (Happy) and is characterized by dense slide patterns that mirror the bouncy melody. The EXPERT chart, designed by 華火職人 (Kanon Oguni), is a 9+ chart with a more balanced distribution of note types. Both lower-difficulty charts (Basic 3, Advanced 6) have no credited designer. The song's steady 148 BPM and cheerful instrumentation make it a standout crossover track in the maimai library.