Sayoko Nanamori is Itsuki's research assistant. She is shown to hold unrequited feelings for Itsuki, and to react angrily if anyone touches on the subject. This leads to a small running gag in the series: Whenever she is particularly upset about Itsuki, the windscreen wiper on her car is shown to make an interval swipe on its own.
Episode 8, "The Dreaming Stone," revolves around Itsuki giving Nanamori a crystal pendant as a Christmas gift; the crystal (taken from the remains of the Dolem Forzando) grows into the Dolem Sforzando, absorbs Nanamori, and alters the weather on Nirai Kanai to create blizzard-like conditions matching the state of Nanamori's feelings. She is freed by Ayato in the RahXephon, and later enters into a relationship with Makoto Isshiki and supplies him with information, thus unwittingly keeping the Bähbem Foundation abreast of Quon's status. Nanamori struggles with memories of her family, but it is later revealed that she was given false memories and was programmed by Bähbem to love Itsuki. Nanamori ends up killing Itsuki by stabbing him. Distraught over Itsuki's murder and the truth of her false memories, she kills herself.
In the movie, Nanamori appears only in a glimpse. In the manga, Nanamori is a malevolent Mu agent and facilitates Ayato's escape from TERRA.
