Kei Shirogane is Miyuki's younger sister who is the student council treasurer in the junior high of Shuchiin. Kaguya admires her because she is much like her brother as she is fairly serious; that being with her is like being with Miyuki. After her parents separated when she was young, she initially stayed with her mother but ran away to live with her father and brother. She is rebellious when it comes to interacting with her brother, finding him to be annoying and not fashionable but still cares for him and ensures that he presents a good image when he is in front of her classmates. She is good friends with the Fujiwaras, both her classmates Moeha and Chika. Despite not interacting with Kaguya much, Kei tries to get to know Kaguya better and is happy when she makes little breakthroughs.