Chisato Hanai is a Level 2 Telepath attending school alongside "The Children," notably classmate Kaoru Akashi. Her childhood friendship with Masaru Tōno fractured when her awakening telepathic abilities unintentionally breached his private thoughts and emotions, creating distance. This rift was later healed through Kaoru Akashi's intervention. Following their reconciliation, Chisato and Masaru's bond deepened significantly, actively nurtured by the encouragement of "The Children."

Recognizing the significance and potential implications of their evolving relationship, B.A.B.E.L. monitors Chisato and Masaru under a formal arrangement. This program was established at the strong request of "The Children" and Fujiko Tsubomi, receiving explicit approval from Japan's sitting Cabinet. Its purpose is to observe and manage the development of their bond within the context of esper-human interactions.

Her name originates from "Hana-chiru Sato," a chapter in *The Tale of Genji*, and references a character within that story, specifically a lover of Hikaru Genji. The surname "Hanai" combines the characters for "flower" (花) and "well" or "pit" (井).