Izumi Kazuko was a 24-year-old science researcher in the cryogenics laboratory with Dr. Sakaki (a.k.a. Dr. Lee). When Jirô and Yurika came to visit the lab, it was Kazuko who showed them around. When the earthquake struck and Jirô became trapped in one of the cryogenic chambers, she decided she and Yurika should take refuge in the chambers aswell.
When they awaken in the future they find a land ruled by apes, with only one known human, Gôdo. She tries to protect the children from the murderous Gebâ as best she can (acting courageously, in spite of her fear) and even suggests they stay in the custody of The Minister, who says he will protect them. Though she has to be persuaded to escape, she forms a close bond with Gôdo during their adventures, and when they are returned to their own time without Gôdo she is upset. She runs to the chamber he was in and retrieves his necklace. Afterwards, she is seen to be still friendly with the two children, buying them ice-cream and explaining what had happened to them and to Gôdo.
In the heavily-edited and English-dubbed version, 'Time of the Apes', she was renamed 'Catherine'.