最終鬼畜妹・一部声 (Saishuu kichiku imouto ichibu koe) is an a capella arrangement by ビートまりお (beatMARIO) that joined the maimai lineup with the ORANGE PLUS update on November 12, 2015. It originally appeared on beatMARIO's 2013 album とうほう☆あまねりお (Touhou☆Amanerio), co-performed with vocalist Amane. The track dispenses with instrumental accompaniment entirely, letting layered vocalizations carry the melody, a stylistic choice reflected in its title, where 一部声 (ichibu koe) means "partial voice" or "vocal part." The song stands at the end of a well-known Touhou arrangement lineage. ZUN's original theme "U.N.オーエンは彼女なのか？" (U.N. Owen Was Her?), the extra-stage boss theme for Flandre Scarlet in 東方紅魔郷 ～ the Embodiment of Scarlet Devil, was first reworked into the high-speed electronic arrangement 最終鬼畜妹フランドール・S. beatMARIO then transformed that version into an a capella reimagining, stripping away synthesizers and percussion to foreground voice-driven rhythm and energy at a relentless 200 BPM. In maimai, the song has seen several difficulty revisions across versions. Its Master chart (13+, constant 13.9, designed by rioN) stands out for its heavy reliance on Slide notes, 164 out of 841 total, creating rapid, sweeping patterns that demand precise multi-finger coordination. The Expert chart (11+, constant 11.6, designed by はっぴー) offers a similarly high-energy experience at a slightly more approachable level. Both charts have been re-leveled multiple times since ORANGE PLUS, reflecting the song's enduring place in the touhou category rotation. The song also appears in SEGA's CHUNITHM, where the maimai version notably omits the fade-out present in its CHUNITHM counterpart.