Many friendships, rivalries and grudges help define My Hero Academia, with Izuku Midoriya's idolization of All Might being just the start. Ochaco Uraraka looks up to Izuku as her own role model, and even the teachers of the U.A. school are connected in more than one way. For example, Class 1-A's homeroom teacher, Shota Aizawa, has an interesting history with Midnight.
These two are certainly not romantically connected, but they have impacted each other in other intimate ways. Students like Izuku would hardly believe that the stoic, stern Aizawa and the fun-loving Midnight have a strong connection, but they do. And that bond helped make Shota Aizawa the hero he is today.
Shota Aizawa & Midnight's Student Days Together
At some point, as shown in the My Hero Academia Vigilantes spinoff, Midnight was a third-year U.A. student while Shota Aizawa, Present Mic and a friend named Oboro were second-years. Even then, Aizawa and Midnight acted the way they would in the main series, but still, they soon became allies and teammates. Midnight, who worked for His Purple Highness' hero agency, offered to let Aizawa, Oboro and Present Mic join as interns -- an offer the boys accepted. At the time, Aizawa was struggling somewhat with his Erasure Quirk, but in the crucible of combat, he would learn to master it. Hitoshi Shinso would repeat this process years later.
During this time, Midnight bonded with Aizawa over their passion for cats, and Midnight helped look after a stray that Aizawa found, naming it Sushi. Midnight and Aizawa were devastated when their friend Oboro was killed in combat, but they had the mental strength and resolve to move on. Later, as a U.A. alum and a Pro Hero, Midnight resolved to become a teacher, and she urged her friend Aizawa to do the same. Aizawa resisted at first, being the bitter loner type, but at some point, Midnight actually submitted the application paperwork to U.A. for Aizawa, and Aizawa finally decided to become a teacher for real. He was rather grumpy about this at first, but Midnight's gung-ho attitude and optimism rubbed off on him just enough so Aizawa gave it a shot. It was a good decision.
Eraser Head & Midnight, Mentors To Class 1-A
Shota Aizawa became the homeroom teacher for Class 1-A, but curiously, creator Kohei Horikoshi once considered putting Midnight in the role, before deciding that a cool-headed character like Aizawa would contrast well with the rambunctious students. In fact, at a few points in the story, Midnight temporarily took over Class 1-A when Aizawa was busy elsewhere. The best example was when Midnight helped the students figure out their hero names -- and she was much more supportive and cheerful than her old friend Aizawa would have been. (Not even Midnight could bring herself to approve of "King Explosion Murder," though.)
As a whole, this friendship is about Midnight bringing out the best in Aizawa, and Aizawa repaid her by being the best possible teacher for the powerful students of Class 1-A. For a time, Shota Aizawa suffered from self-doubt and a lack of initiative, until Midnight and Aizawa's other allies instilled some confidence in him and encouraged him to care about other people, not just himself. Shota Aizawa wasn't exactly on the path to becoming a villain, but he was quickly hitting a wall as a hero, and as a person. Midnight helped him vault right over it.
Now, in the present, Aizawa finds teacher work highly rewarding, and this grouchy mentor takes great pride in his students, will do anything to protect them, and brings out the best in them -- just as Midnight once did for him. It's the best way Aizawa can repay Midnight for the positive influence she's had on him.