The following contains spoilers for My Hero Academia, available to stream on Crunchyroll and Hulu.
My Hero Academia features some of the most interesting characters in anime. Aside from the captivating villains that have been introduced in the series, many of the characters with notable character development and immense strength attend (or have attended) U.A. High School. Such characters include the Big 3, Deku, Bakugo, Best Jeanist, and Edgeshot.
Of the many U.A. graduates over the years, several heroes chose a path unlike their colleagues: teaching. U.A. High School recruited some of the best Pro Heroes (and former Pro Hero, All Might) as teachers, including Midnight, Present Mic, Vlad King, and Eraser Head. Each Pro Hero is assigned their own Homeroom course or subject to teach (according to their personal expertise). In addition, most of these Pro Heroes are quite famous in Japan. However, one individual specifically stands out for his introverted demeanor: Eraser Head. But who is Eraser Head, and why is he not in the public eye more often?
As a U.A. Student
Like most Pro Heroes, Aizawa attended U.A. High School and was placed in Class 2-A. There, he met Hizashi Yamada (Present Mic) and Oboro Shirakumo (Loud Cloud), and they became inseparable. Their close friendship was evidently so special that Hizashi was the one who gave Aizawa his hero name Eraser Head. His friends' bright dispositions balanced his own impassiveness. They both inspired and propelled him to improve, even if they didn't realize it. For example, Oboro encouraged him to be a better hero and gave him his goggles, which Aizawa began wearing to benefit his own quirk. In addition, the trio had plans to open their own Hero Agency, but this never transpired.
Tragedy struck Aizawa's life (as well as Hizashi's) when Oboro was killed by a villain during their Hero Work-Studies with His Purple Highness. Following the loss of one of his best friends, Aizawa isolated himself even more. He also upped his training and was determined more than ever to graduate and fight villains as Eraser Head.
Career
As the homeroom teacher for Class 1-A, Aizawa is responsible for keeping an eye on the kids and coordinating them. Unlike his colleagues, Aizawa seems to only really teach general hero knowledge to his homeroom. In other words, he does not have a specific subject to teach, like Present Mic, who teaches English, due to his bilingualism. Rather, Aizawa must ensure his students are excelling and passing. When his students move to dorms on campus, he is also responsible for them as their primary guardian after school hours. His job as the homeroom teacher for 1-A matches perfectly with his personality.
Teaching his students at U.A. High School is not Aizawa's only job, however. In the evening, he works as an underground Pro Hero. This is because he prefers living a private life, which he couldn't have if he worked in the spotlight.
Personality
Aizawa is quite reserved and tends to make decisions based on logic instead of emotions. He prefers to be alone, having worked alone for years. He also does well in keeping his composure and usually comes off as apathetic and lazy (and prefers to nap in his yellow sleeping bag). Needless to say, Aizawa is blunt and, quite honestly, looks the part with his unkempt appearance. Although he proves he doesn't care about a lot of things that are insignificant to him, Aizawa is not afraid to speak out about his opinions and ideas.
Based on his personality, he doesn't seem like the type of person who cares about anything. However, this is quite the opposite. Aizawa is very caring, but he does not show it through emotions. For example, as a teacher, he pushes his students to become better heroes through severe training and the harsh reminder that heroism is not all that it seems. When Shigaraki and the group of villains attacked U.A., Aizawa defended his students against multiple villains by himself. This easily exhibits the extent to which he cares about his students.
Quirk
Aizawa's quirk is called Erasure, which means he is able to temporarily erase another person's quirk just by looking at them. It's a powerful and extremely useful quirk, but like every other quirk, it has its flaws. For example, his quirk stops working when he blinks. This means Aizawa has had to learn to keep his eyes open for a long time. This also implies that his eyes become dry quite frequently (which is why he is seen carrying eye drops with him).
In addition, when using his Erasure quirk, his eyes turn red, and his hair floats, making it obvious when he is using his quirk. At one point, when he was a student at U.A., Aizawa was questioning whether he had what it took to become a Pro Hero. After all, his quirk is not meant for offense. Rather, it's meant to assist those with an offensive quirk. This makes Aizawa's overall fighting skills all the more impressive, as he uses his capturing weapon to aid him.