Miyuki Shiba

is the main female protagonist of the The Irregular at Magic High School light novel series. She is the biological sister of the main protagonist, Tatsuya Shiba and just like him, is a student of First High School. The spin-off manga, The Honor Student at Magic High School, has Miyuki as the main protagonist, depicting her point-of-view in a timeline parallel to when the original manga started.

Background
Miyuki is the daughter of Tatsurou Shiba and the late Miya Shiba and is eleven months younger than her only elder brother, Tatsuya (her birthday falls on March 25, a month before Tatsuya's birthday). They are mostly mistaken as twins for being in the same year.

Prior to the timeline, before the beginning of the series, their father remarried six months after their mother's death, and the two of them lived alone in a house, which is described as a place that vastly exceeds the average size. The siblings lived alone at first, however, later on Minami joins them as Miyuki's second guardian.

It is revealed that she is an augmented magician (akin to Sakura Series) which is made especially for Tatsuya to have a life partner, and so that he would not go berserk and destroy the world. According to Maya, much technology was used in her modification, making Miyuki an extremely unique magician. Due to the modification done on her, Miyuki is a unique magician in an entire class of her own and even the Yotsuba will be unable to create another one like her. Compared to Minoru, who also has received similar modification done by Kudou Makoto before birth, Miyuki is a far more perfect and successful product proved by her stable health and lifespan. Because both of them are somehow similar in the aspect of their makings, this is actually the reason why they both have distinctive similarities; formidable magic abilities and striking ethereal-otherworldly physical appearances.

Miyuki is a descendant of the Yotsuba Family and was born with superior and formidable magical abilities. Due to this, she grew up as a pampered child and was raised in a manner befitting one of the candidates to succeed the family's Head position sometime in the future. Like other descendants of prestigious families, she attends the various type of classes with special tutors such as arts and more. Unlike her brother, who was technically demoted from an heir to being a mere servant, Miyuki enjoyed the privileges a Yotsuba deserves. She is now the number one potential successor. In Volume 8 of the light novel series, it is stated how unequally she and Tatsuya were treated by their family and relatives. The Reminiscence Chapter also showed how vastly different Miyuki's opinions and actions towards her brother were three years before the current timeline.

Appearance
Miyuki is described as a lovely, beautiful girl who captivates whoever sets their eyes on her. Her arms and legs are slim, slender without looking too unhealthy. It was mentioned by Mayumi that the number of perfectly symmetrical features in Miyuki was overwhelming to the point that "her confidence in herself wavered." This might be caused by her magic prowess which is very formidable and greatly balanced. Miyuki has noted that her features are not entirely similar to that of her family. Her figure has become more defined and curvaceous over her first two years spent at Magic High and has developed a much more mature appearance in her last year at First High.

Because of her openness, modesty, well-brought up manners and behavior, coupled with her lovely, beautiful appearance, Miyuki is popular not just among male freshmen students of their school, but also among females and upperclassmen, and even to magicians from other schools and outsiders that may or may not follow the Nine School Competitions' news.

Abilities

 * Sensory Abilities: Miyuki has the ability to detect the state of Eidos, or "information," through touch and smell. Not much is known about the extent of her sensory abilities, but she is able to use her senses to "smell" that Tatsuya had been in a fight with over 10 people and was injured in that fight. She also later, used her touch to receive the information from Tatsuya which he captures by his special eyes, in order to let her become informed of the whereabouts of enemies before using her magic to render their weapons useless.
 * Magic:
 * Inferno is a Medium-Scale Oscillation-Type Magic. A magic which reverses the heat energy within a set area. Areas of bitter cold and scorching heat occur in adjacent regions. By dividing the target area into two sections and decreasing the kinetic and rotational energy of all objects within one section, this surplus energy could be released into the other section in the form of heat. This way, conservation of energy is still maintained while applying the principle of reverse entropy from thermodynamics. It is mentioned that this spell is used in the examination to become a Rank A Magicians.
 * Niflheim is a Wide-Area Deceleration-Attribute Oscillation-Type Magic and considered a Rank A Freezing Magic that is able to produce dry ice and even liquid nitrogen from the air. This technique ignores heat and appearance, using magic to uniformly cool down all the objects within a set area.
 * Glacial Age is an ultra-wide area effect cooling magic on par with Strategic Class Magic. The spell is an extension of the Deceleration/Oscillation type wide-area magic ‘Niflhiem’ and invoked using the process of ‘Chain-Casting’ utilized by ‘Tuman Bomba’ and ‘Ocean Blast’. In the situation where it is used in an area of open sea, it reaches up to about twenty kilometers in diameter, and because it requires aiming from such a difficult long distance, it uses an exclusive specialized CAD with the potential to invoke at long distances.
 * Freeze Flame is a Wide-Area Deceleration-Attribute Oscillation-Type Magic that freezes fire. It is a conceptual magic that prevented anything from burning. This magic can suppress the temperature of targets within a certain threshold. Against this spell, most firearms become useless as it becomes impossible to ignite gunpowder and fire projectiles.
 * Zone Interference is Miyuki's Counter Magic. It is used solely to shut down all magic with the exception of your own. Miyuki's Zone Interference is one of the best in the world and is so powerful that even Tatsuya and Lina have troubles under it.
 * Deceleration Zone is a fairly common magic used to slow down movement of objects within a target area. But when Miyuki uses it, the targets extended to gas molecules too. Miyuki utilizes this spell, which is only used in battle as a second option to reduce the impact of projectiles when lacking the power to outright stop them, as a two-fold anti-personnel magic due to her overwhelming magic power.
 * Luna Strike paralyzes the mind by forcibly loosening the bonds of will so that emotions will run rampant through a phantom blow.
 * Cocytus is Miyuki's innate unique magic. Unlike her other Freezing Magic spells, Cocytus doesn't affect physical temperature. Instead, it freezes the mind and spirit. Miyuki is skilled enough with this magic to selectively choose whose minds to freeze even if the target is among a closely-packed people. However, because she serves as Tatsuya's magic limiter, this very unique and powerful ability becomes more available to her when she removes the seal to Tatsuya's magic power.

(Source: Mahouka Koukou no Rettousei Wiki)

Trivia

 * Miyuki appears as a playable character in Dengeki Bunko: Fighting Climax.
 * Miyuki appears as a main and support character in Dengeki Bunko: Crossing Void.
 * Some of Miyuki's spells (such as Inferno, Niflheim, and Cocytus) are named after a certain place or aspect of the underworld or afterlife.
 * An image of Miyuki makes a cameo in the anime adaptations of Durarara!! and Accel World.
 * Miyuki is voiced by Saori Hayami in the anime adaptation of The Irregular at Magic High School.
 * She shares a voice actor with: Suiran Fou (Ladies versus Butlers!), Ayase Aragaki (Oreimo), Fuuka Kamiigusa (The Pet Girl of Sakurasou), Seiko Ugaki (Shakugan no Shana), Sachi (Sword Art Online) and Kou (Our Home's Fox Deity).