|Spoiler Alert: This page contains spoilers. Read at your own risk.|
Appearance and Personality
Kasumi is a bit of a tomboy with a bob-like hairstyle without any curls, and two small ribbons. Despite looking identical to her twin, Izumi, they are polar opposites in personalities.
When meeting Tatsuya for the first time, she assumes that he is after her elder sister Mayumi and calls him a "skirt chaser". She radiates an aggressive aura towards him, attempts to use magic on him, rejects any chance of familiarity between them, is jealous and wary of him and accepts her position in the Public Morals Committee in an attempt to show him up.
She is suspicious of any man who gets near Mayumi, shown not only by her reaction to Tatsuya, but also by Hattori's reaction when seeing her name on the list of candidates to join the School Council.
Kasumi and Izumi both enrolled as Freshmen students of the same school in the year 2096, and came in as third and second place, respectively, during the entrance examinations.
With her twin sister and Mayumi, they shared a similar sharp sensitivity toward magic, it means that the practice was superior to her ability with theory. With Izumi, their ability to discern the identity of invoked sequences might be even greater than Mayumi’s own.
Izumi and Kasumi also bear the ability to "Resonate" with each other. This means that when one uses magic, the other's Magic Calculation Area is also stimulated. The spells they cast together are stronger than the ones they cast separately, leading them to be given the reputation that "the Saegusa twins can only reveal their true strength when they are together". Kasumi and Izumi can increase their Magic Power simply by using normal magic through the usual assistance of their CADs. In addition, the two of them are not components of a Magic Sequence but combine to form a single, greater Magic Power. This is called as "Multiplicative Magic".
Kasumi loves to hurtle Magic Sequences at her target while Izumi gives the spell interference strength. Their Magic Power is multiplied rather than added together when casting magic. To be able to accomplish this, not only do their bodies have to contain the same hereditary genes, but their mentality and unique Magic Calculation Areas have to be the same. Takuma might have determined that Kasumi was stronger in invocation speed and scale of design for Magic Sequences while Izumi was stronger in interference strength; this is actually a misunderstanding. The reason he was led to believe this was because that is how Kasumi and Izumi choose to wield their magic, so the result would be the same even if they swap roles.
- Suffocating Turbulence (窒息乱流)
- On the level of neutralizing an opponent by using magic to induce a lack of oxygen, this spell probably belonged to the same category as Mayumi's anti-personnel trump card "Dry Meteor". Kasumi and Izumi probably devised this trump card due to Mayumi's influence. In terms of difficulty, Suffocating Turbulence is far more difficult than Dry Meteor. Though it is a hassle to gather a large amount of dry ice particles laced with carbon dioxide in the air beforehand. Suffocating Turbulence requires constant maintenance of the molecular composition in the air as well as careful control of the air currents, rendering this an extremely difficult magic. 
- It targets air molecules while gathering and moving them. It creates a furious tornado that attacks the target from all directions. Even with a sealed barrier protecting against all directions, the wind affects the target.
- This is a magic that raises the level of nitrogen in the air as well as magic that moves blocks of air around. It is a combination of Convergence-Type and Move-Type Magic. A single breath of this air that is heavily deficient in oxygen was immediately render someone unconscious due to the lack of oxygen. If a barrier is lowered to prevent the target from being blown away, then they would be suffocated.
- Despite the two of them being able to use such a high level magic that cannot normally be seen in a high school setting, Kasumi and Izumi's control of the air currents for this spell is still far too coarse.
- Heated Turbulence (熱乱流)
- By slamming blocks of air with large amounts of oxygen into the attacking paper snowflakes, they were using adiabatic compression to create heated wind that surpassed the paper's burning point to ignite the paper blades. 
- This is an alternate form of "Heated Turbulence". This is a type of magic that is another level higher than simply creating adiabatic compressed air blocks. Kasumi and Izumi can activate this high level magic while Suffocating Turbulence is ongoing due to the boundaries of their Magic Power when they are Resonating together. Due to their being from the 3rd Institute originally, for the Saegusa, even if it is high level magic, using two or three of them at the same time is a simple task.
- Freeze Air Bullet (凍気弾) -
- Air is cooled down and compressed, then used as a bullet as it loses its heat. It is more similar to Mayumi's favorite magic, “Dry Meteor”, but instead of using carbon dioxide, it just uses air Although the compound element using nitrogen and oxygen as the main ingredient has lowered the freezing point lower than carbon dioxide, the compressed bullet has a different effect than dry ice bullet.
- Nitrogen Storm -
- In this magic, air which consist of 90% nitrogen blows towards the target. Due to this, oxygen concentration around the target reduces to an extreme which results in induce oxygen-deficiency syndrome.