Hinoki Sai  

, 彩火乃紀

The daughter of a renowned paleontologist who disappeared during an expedition in India four years before the show's start, Hinoki was orphaned and later recruited by Akamatsu and Mode Warp as a Head Diver. She often appears sullen and frequently belittles her own intelligence when faced with questions she cannot or does not wish to answer. Despite being a childhood friend of Keita, she had not seen him in some time before their reunion in the first episode. Although Hinoki typically shows disinterest in his romantic advances, there are instances where she reveals that she cares more for him than she lets on, referring to him affectionately as "Kei-chan". Like Keita, Hinoki is seventeen at the start of the series.