The Irregular at Magic High School takes place in an alternate history world in which magic exists. After the third world war that lasted 20 years and destroyed a large amount of the human population, the planet was divided into four major superpowers with Japan being one of the four superpowers. As a country, Japan is ruled by the Ten Great Clans, one of which is the Yotsuba Clan, which co-protagonist Miyuki Shiba will lead in the future.
Much of The Irregular at Magic High School focuses on Miyuki's relationship with her brother Tatsuya, who is also her bodyguard. The “Flashback Arc” pauses the present timeline to explore the brothers' origins three years before the “Registration Arc” of Part 1. While Miyuki is very attached to Tatsuya in the present, the “Renaissance Arc” ” reveals that this is not always the case.
Before becoming aware of her brother's strength and magical talent, Miyuki did not appreciate Tatsuya and treated him like a servant, just like their mother and the rest of the Yotsuba Clan did. Although Miyuki was not cruel to him, she was also not affectionate; she just left him alone to perform his duties as a family servant. What she had a problem with was letting him go with her everywhere she went – because he scared her. This was most noticeable during a family trip to Okinawa, when she wanted to go out for a walk and enjoy the beautiful scenery.
Tatsuya stayed behind Miyuki while she walked, which made her feel uncomfortable. At first, it seemed like her annoyance came from contempt, but it quickly became something else. Miyuki wondered why her brother was not respected like the rest of the Yotsuba clan. She also didn't understand why she was wondering this, since she had been raised not to think of Tatsuya as an older brother or address him as such.
It's not until they come across some Left-Bloods – soldiers who are descendants of American soldiers after World War III – while walking that she begins to realize Tatsuya's true personality. Realizing that soldier Joe Higashi is a threat to Miyuki, Tatsuya quickly intervenes and engages him in hand-to-hand combat, winning easily. This was enough to impress Miyuki, only adding to her conflicted feelings towards her brother.
As Irregular's “Reminiscence Arc” continues, Miyuki finds herself feeling sorry for her brother, which quickly escalates into feelings of love. Despite her newfound feelings, she still struggles to understand why he only feels obligated to her and nothing else. This confusion especially occurs when she tries to save him for her mother's bodyguard, Honami Sakurai. When he considered protecting Miyuki as just doing his job, she felt a little hurt.
During a conversation with Honami, Miyuki learns that as the heiress of the Yotsuba Family, she must get used to not hiding her emotions, as that could make her easily mistaken for someone and do something wrong. damaging his position in the Family. Despite this sound advice, Miyuki still couldn't help but feel a little jealous when someone interacted with Tatsuya at her cousin Mitsugu Kuroba's dinner party, especially when she saw him smiling sincerely—which he never did for her.
On the eve of the Greater Asian Alliance's attack on Okinawa, Miyuki, her mother, and Honami were all shot down by intruders at Onna Base. They were only saved when Tatsuya revived all three and destroyed the intruders. Since protecting Miyuki's life was Tatsuya's responsibility, he accompanied the soldiers of Onna Base to fight the Greater Asian Alliance as payback for harming his sister. These actions eventually solidify Miyuki's love for Tatsuya to the point of asking their mother why he feels obligated to her.
The brothers' mother, Miya, reveals that after surgery to enhance his magical abilities, one of the side effects caused him to lose his emotions, all except one: love for you. my daughter. From that moment until the present at Magic High School, Miyuki is committed to her brother – even when she knows she cannot be with him romantically due to blood ties.