|Full Name||Saegusa Kasumi|
|School||First High School|
|Year/Course||Year 1 Class C Course 1 - AD 2096|
|Voiced By||Eri Kitamura (Lost Zero)|
|Novel||Volume 10, Chapter 10|
She is incredibly close with her twin sister, Izumi. Izumi often helps Kasumi when she gets into trouble due to losing her temper.
Kasumi shares a good relationship with her father and is close to him.
Kasumi looks up to her older sister Mayumi with respect and loves her very much. However Kasumi has shown that she slightly fears Mayumi as well, if Kasumi ever greatly angers or annoys her. Kasumi shows concern that Mayumi is wasting her youth because she doesn't have a boyfriend yet. However Kasumi has shown that she is very protective of Mayumi, to the point she would try to interfere when boys appear to be trying to get close to her and possibly scaring them away.
In Volume 17, Kasumi protests Kouichi idea of Mayumi pursuing a romantic relationship with Tatsuya because it will ruin Mayumi reputation and Mayumi would be view as seducer in other people's eyes.
It is revealed in the conversation of the Saegusa sisters in Volume 11 (Visitor III), Chapter The Ojou-sama's Magnificent Day off, that the twins either scared or threatened him, personally or more probably by an intermediary, for trying to approach or court Mayumi.
Kasumi probably has the same hostile attitude towards him that she bears towards Tatsuya.
Kasumi showed Tatsuya much hostility the first time they met because she thought that Tatsuya was flirting with Mayumi, and tried to attack him. Kasumi is unfriendly towards Tatsuya, and is greatly angered whenever Tatsuya approaches Mayumi. It is stated that Kasumi thought that Tatsuya was not popular with his female classmates, but changed her mind after seeing Honoka giving a birthday present to him.
It is speculated that she may have developed romantic feelings for Tatsuya from that time and also demonstrated that she is jealous when Tatsuya takes Pixie to Nine Schools Competition. Katsuto once suggested to Tatsuya that he marry one of the Saegusa sisters to gain membership to the Ten Master Clans.
After the Nine Schools Competition of AD 2096, Kasumi starts to feel respect for Tatsuya as a senpai.
In Volume 17, Kouichi reveals that Tatsuya and Miyuki are engaged and both of them are from the Yotsuba, which shocks Kasumi. Kouichi then asked her what she personally thought about Tatsuya. Kasumi simply said that she didn't know Tatsuya very well, but she knew that he is talented in magic engineering and sensed that he has formidable power.