WARNING: The following contains spoilers for The Irregular at Magic High School, "Reminiscence Arc," now streaming on Funimation and Crunchyroll.
Tatsuya Shiba and his sister Miyuki are the protagonists of The Irregular at Magic High School. Despite being her older brother and a powerful magician in his own right, Tatsuya is treated as nothing more than a servant by his family except for Miyuki. Although he possesses impeccable combat skills and strong control of magic, he is still placed in Course 2 when he officially enrolls at the National Magic University Affiliated First High School.
While Tatsuya is well-liked and respected by his sister and classmates, much about him remains a total mystery. For example, he is never bothered by the way other people treat him and seems overwhelmingly accepting of his position in life. Ironically, despite his seeming lack of emotion, he is highly protective of his sister and maintains a very intimate relationship with her, much to the discomfort of everyone else in their inner circle. While the prior two seasons of Irregular didn't reveal much about his past, the "Reminiscence Arc" fills in a lot of gaps.
Three years prior to the events of Season 1's "Enrollment Arc," a very young Tatsuya accompanied Miyuki and their mother on a trip to Okinawa. While the latter two flew first-class, Tatsuya was seated with the plane's economy passengers. He was also in charge of picking up their luggage and escorting them to their vacation home like a bellboy. At the time, Miya was the head of the Yotsuba Clan instead of her sister (and the present-day head) Maya. Despite being treated as a servant by his mother and sister at the time, even back then Tatsuya didn't reveal any feelings of anger or degradation.
Throughout The Irregular at Magic High School's "Reminiscence Arc," Tatsuya continues to show his potential as a powerful magician. He even impresses Miyuki with his prowess as a hand-to-hand combatant when they encounter Left-Bloods -- descendants of US soldiers post World War III -- during her walks. Despite his highly protective behavior, it puzzles Miyuki that he sees his own nature as nothing more than his responsibility. This leaves her feeling a little hurt and even a bit jealous of her cousins when he genuinely smiles at them at a dinner party. Not being able to reconcile his behavior with his emotions, Miyuki at one point asks Tatsuya about this inconsistency. He tells her to ask their mother about that.
While Tatsuya is fending off an attack against Okinawa by the Great Asian Union, Miyuki asks her mother Miya about why her brother behaves the way he does. Miya agrees to tell her about his origins -- on the condition that she stops treating him like a brother. It turns out Tatsuya was born a defective magician, completely unfit to lead the Yotsuba Clan. While normal magicians have the capacity to manipulate and transform matter, Tatsuya is only capable of disassembling and reassembling objects. This posed a problem for the Yotsuba Clan, but they still found some use for his strange power.
Deciding he could be useful as a bodyguard, both Miya and Maya performed a magical surgery on Tatsuya to augment his existing powers. This surgery is similar to other modified magicians who have also had their genetic information altered. While the procedure was a success, it also cost Tatsuya his ability to experience emotions such as anger, grief and jealousy. He isn't even capable of feeling any love toward his own family members, except Miyuki. Miya rationalizes that if he has to feel affection for someone, it should be the person he'll be assigned to protect.
Learning about the surgery her mother and aunt performed on Tatsuya proves too painful for Miyuki to hear, but it does help bring closure to her conflicted feelings toward him. The Irregular at Magic High School's "Reminiscence Arc" not only explains why Tatsuya behaves the way he does in the present, but it also explains Miyuki's own attachment to him, however discomforting it is to the people around them.