Kei Shirogane is Miyuki Shirogane's little sister. She attends Shuchi'in middle school and is the treasurer for the middle school student council.
She is a hard worker just like her brother and can be very prideful at times. She also inherited his sense of righteousness and hates it when someone doesn't do the right thing in a given situation. Like her brother, she likes helping out those who are in trouble even if it means going out of her way to do it. Since she grew up in a family that places emphasis on saving money, she normally carries coupons and works hard for the things she wants to buy.
She developed a prickly attitude towards her brother due to his over protectiveness. She's also entered a rebellious phase and due to her pride and she cannot get herself to have a lighthearted conversation with Miyuki, leading into the similar mind games that Miyuki and Kaguya play against each other, but her objective is to talk to Miyuki without making it look like she has a brother complex. She rarely speaks and most of the time it is only when she exchanges formalities at school or when she hurls abuses at her brother.