Kiyomi Otaka built an independent solo career for fourteen years — starting with Third Hand in 1998 — before joining Casiopea on keyboards on April 20, 2012, replacing founding member Minoru Mukaiya in the lineup now known as Casiopea 3rd (later Casiopea-P4). She is the only female member Casiopea has ever had, and one of only two members, alongside bassist Yoshihiro Naruse, who arrived with an established solo career already behind them rather than building one inside the group.

Her keyboard voice is technically distinct from her predecessor's: where Mukaiya was associated with synthesizer-driven keyboard textures, Otaka started as an organ player, and that grounding is audible across the run of records she's made with the band since — Ta・Ma・Te・Box (2013), A・So・Bo (2015), I・Bu・Ki and A・So・N・Da (2016), A・Ka・Ri (2018), and, under the Casiopea-P4 name, New Topics (2022) through P4's Finale Sound Version (2025).

She has kept a distinct bandleading identity of her own throughout her Casiopea tenure. In 2013 — the same year Casiopea 3rd's first album was released — she co-founded Kiyo*Sen, a fusion jazz duo with drummer Senri Kawaguchi, which had released five studio albums, two live albums, a single, and two live DVDs by 2023, and continued with a 10th-anniversary compilation in 2024.

Otaka's organ-rooted approach distinguishes her contribution to Casiopea's sound from the synthesizer-forward keyboard work of founding member Minoru Mukaiya, while her Kiyo*Sen project develops a more intimate, duo-scaled fusion voice alongside drummer Senri Kawaguchi.