Hinoki Sai

The daughter of a famous paleontologist, who was orphaned when her parents and older brother vanished during an expedition to the Ajanta Caves in India four years before the start of the series. She was recruited as a Head Diver by Akamatsu and Mode Warp shortly thereafter. Hinoki acts in a sullen manner a majority of the time, and often derides her own intelligence when asked questions that she can not, or does not wish to, answer. She is a childhood friend of Keita, but had not seen him in some time before their reunion in the first episode. Although she usually acts uninterested in his romantic advances, she shows on several occasions that she cares more for him than she lets on.

She also refers to him affectionately as "Kei-chan". She, like Keita, is seventeen when the series begins.