"Ontology" (オントロジー) is the title track from Rain Drops' second mini-album, released in 2020. Rain Drops is a vocal unit formed from VTubers affiliated with ANYCOLOR Inc. (formerly Ichikara), bringing together performers such as Ryushen, Akina Saegusa, Meiji Warabeda, Masaru Suzuki, Elu, and Joe Rikiichi. The track was composed, arranged, and written by Jin, who also contributed guitar, a familiar name to maimai players from his earlier works "Kagerou Days" (カゲロウデイズ) and "Additional Memory" (アディショナルメモリー), both of which appear in the maimai song library. Bass was handled by Shota Horie (PENGUIN RESEARCH), drums by Yumao (hitorie), and the music video was directed by RIRI Yamashita. The title "Ontology" is a deliberate philosophical reference: in Western philosophy, ontology is the study of being and existence. For this song, the word is repurposed to mean "existential proof" for VTubers, a meditation on identity, presence, and what it means to exist as a virtual performer. The lyrics paint a picture of standing in the rain under mismatched umbrellas, searching for a shared sky, which resonates thematically with Rain Drops' collective identity as individual personalities forming a single unit. In maimai, "Ontology" was added as part of the DX UNiVERSE PLUS update on June 10, 2022, and was one of several songs later removed from the arcade rotation starting in March 2025 with the transition to PRiSM. The song's four DX charts span from a Basic level 3 to a Master level 12+ (constant 12.8), with Expert charted by Jack and Master charted by Happy (はっぴー). The Master chart is notably slide-heavy, with 149 Slide notes out of 702 total.