Rui Nikaidō, the 18-year-old lead singer and center of idol group Mystery Kiss, combines exceptional singing and dancing skills with striking physical presence. Her personality is fiercely competitive, driven by an aversion to defeat and a relentless ambition to surpass others.

Founded in 2019 and managed by Fuyuki Yamamoto, Mystery Kiss originally included Rui alongside Shiho Ichimura and Yuki Mitsuya. Around 2020, Rui entered a relationship with Atsuya Baba of comedy duo Homosapiens. Amid financial pressures and personal ambition within the idol industry, Rui engaged in illicit activities orchestrated through yakuza ties, including romance scams targeting affluent men.

Rui viewed backup singer Yuki Mitsuya—whose athleticism and talent threatened her center position—as a rival. This tension escalated when management contemplated replacing Rui as lead vocalist. After Yuki’s disappearance and murder by Sakura Wadagaki (who later impersonated her), Rui retained her role without direct involvement in the crime.

Taxi driver Hiroshi Odokawa’s visual agnosia, which made him perceive humans as animals, initially rendered Rui as a toy poodle. This depiction stemmed solely from Odokawa’s condition. Upon his recovery in the series finale, Rui’s human form was confirmed.

Following Sakura Wadagaki’s arrest and the resolution of the missing girl case in "Odd Taxi: In the Woods," Rui continued performing with Mystery Kiss. The film’s credits verify the group’s ongoing activities post-events, though Rui’s specific developments during this period are not detailed.