The Irregular at Magic High School (aka Mahouka Koukou no Rettousei) is a world with an alternate history in which magic exists. People are categorized based on their magic abilities and talents, with the more powerful magicians being favored. This discriminatory system not only impacts how people are valued within human society but also by their own families, as explored in the "Reminiscence Arc." One of the main characters impacted by this system is Tatsuya Shiba, who was born a "defective magician" and was treated as a servant by his own mother and sister at first.
When Tatsuya and his sister Miyuki enrolled as students at First High School, the same discriminatory system that rigidly defined their childhoods followed them into their school placement. As the reputed more powerful magician who is set to succeed Maya Yotsuba as the leader of the Yotsuba clan, Miyuki is placed in Course 1, aka "The Blooms." As the irregular or "defective magician," Tatsuya is placed in Course 2, aka "The Weeds." Despite being highly talented for his labelling, Tatsuya is treated the same as all the other "weeds" by the "blooms" -- until he gets noticed by Mari Watanabe.
Mari is something of an interesting character during Mahouka's "Enrollment Arc" in Season 1. As the chairman of the Public Morals Committee, Mari holds quite a lot of weight around the school and is highly respected by the Course 1 students. When she invites Tatsuya to join the committee, she catches the others -- including Tatsuya himself -- completely off guard and is even met with opposition, most notably by Hattori Gyoubushoujo Hanzou.
To prove a point about Tatsuya's lack of qualifications, Hattori challenges him to a duel, which Mari allows. Not only does this show that Mari has full confidence in Tatsuya's abilities, but her decision to recruit him is subsequently validated. Hattori's overconfidence in his status as a Course 1 student clouds his judgment, which Tatsuya uses to his advantage and defeats him with ease. Beaten and humiliated, Hattori reluctantly accepts Mari's nomination of Tatsuya, though he continues to look down on him.
While Tatsuya is not accepted by the Course 1 students right away, Mari inviting him to join the Public Morals Committee still proves to be an important first step to Tatsuya climbing the school's ranks. Not only does he start making friends with both the Course 1 and Course 2 students as the story progresses, but Mari's actions also facilitate a "bridging of the gap" between the two bodies of students, leading to a more unified school. Of course, it's not just this one action that helps to unify the factions, but Mari's personality traits, personal values and leadership skills play a huge role.
A bit later in Mahouka Season 1, Mari vocalizes her disgust over First High School's discriminatory system and practices. Despite being a Course 1 student, she proves to be highly perceptive to everyone's well-being, finding this system so limiting of students' potential that she becomes determined to change it. One way she achieves this is by functioning in an "older sister" role to the other students, even if she doesn't like being referred to as such by her peers.
As a testament to her strong leadership skills, Mari is shown to be sympathetic toward others but can be tough-as-nails when she needs to be. She also vocalizes her thoughts when something doesn't sit right with her, and doesn't let the opinions of other students affect her decisions. These traits are highly admired by other Course 1 students, most notably her best friend and Student Council President Mayumi Saegusa. Additionally, whenever incidents take place at the school involving Course 2 students, Mari is among the first to vouch for them and clear their names of any wrongdoing.
Without this combination of traits, Mari would not have successfully pulled off the kinds of changes First High School experienced during her time in a leadership position. While controversially inviting Tatsuya to the Public Morals Committee may have initially catapulted the changes Mari wanted, it was ultimately her ability to stick to her values and lead by example that led to changes in attitudes between Course 1 and Course 2 students, resulting in a more unified school.
