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Saegusa Izumi (七草 泉美) is one of Mayumi's younger twin sisters who both had cameos in the WEB Novel Arc 5 & 10, in a scene where they watched their sister make bitter Valentine's chocolate for Tatsuya and Hattori.
Izumi, along with her twin Kasumi, are formally introduced in the released segments in Dengeki Bunko Magazine from an arc that features the Shiba siblings' sophomore year in First High School called "Double Seven", when the twins both enrolled as freshman students.
Izumi ranked second during the entrance examination, and joined the Student Council.
Appearance and Personality
Izumi is described as a feminine girl with hair in a straight bob that goes down to her shoulders and bangs that reach her eyebrows. She is noted to be an indoors type person or a bookworm.
Izumi usually conceals her thoughts and feelings behind an unreadable and gentle smile, and she rarely lets her emotions run wild.
She developed a strong crush on Miyuki, whom she admires as the perfect "Yamato Nadeshiko"  and a goddess. She even asks to call her "Onee-sama" in the first meeting, and even joined the Student Council just to be with Miyuki.
The plot doesn't give an exact answer to the question, whether Kasumi see Miyuki as romantic interest or just admire her as an ideal role in akogare way, but in volume 17, its briefly mentioned that her father knows that "she is attracted to the Miyuki's beauty".
She seems to be more knowledgeable than her sister Kasumi, knowing the information about Tatsuya's involvement in the Nine Schools Competition. She also appears to respect him.
Out of the twins, Tatsuya sees her as the one to be the most wary of.
It's widely known among magic officials that Izumi, whom is the youngest daughter of the Saegusa Family, is liked best by her father Kouichi. 
With her twin sister and Mayumi, they share a similar sharp sensitivity toward magic, meaning that the practice is superior to her ability with theory. With Kasumi, 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. It 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. Shippou 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)
- Also known as "Nitrogen Storm" is 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 would 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 Research 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.
- Cold Storm (冷気嵐流) -
- Izumi created this new magic because of her admiration towards Miyuki and her freezing magic. Here,The air is compressed in an instant, while not varying its current temperature. That is, when it is released, due to adiabatic expansion, the flowing air creates sudden temperature drop, resulting in effectively freezing the target. 
- Is a magic that creates a zone with increased oxygen concentration.