The character of Shota Aizawa, known as the underground hero Eraserhead, is depicted in My Hero Academia: Vigilantes during a period before he became a full-time teacher at U.A. High School. This prequel story reveals a younger, more active hero at the start of his professional career, showing the experiences that shaped his later persona.

Prior to the events of Vigilantes, Aizawa was a student at U.A. along with his close friends Hizashi Yamada and Oboro Shirakumo. During his second year, he struggled with significant self-doubt about his ability to become a true hero, an insecurity highlighted when he encountered an abandoned kitten on a rainy day. Feeling powerless to save it, he could only leave his umbrella to shield the animal. The loss of his friend, Oboro Shirakumo, who died in an incident during their school days, was a formative tragedy for Aizawa. This event steeled his resolve but also instilled a more solitary and pragmatic approach to hero work. It was also during his school years that his friend Hizashi gave him the hero name Eraserhead.

In Vigilantes, which takes place roughly five years before the main series, Aizawa is shown as an active pro hero who performs his duties with a serious, no-nonsense attitude. He works primarily as an underground hero, preferring to operate away from the spotlight and media attention. He has little tolerance for what he considers irrational behavior and maintains a perpetually exhausted demeanor, characteristics that would later define his teaching style. Despite his gruff exterior, he is driven by a strong sense of duty to protect civilians and stop those who misuse their powers, often putting himself in harm's way to achieve this. A pivotal motivation for him during this era is investigating the criminal organization known as the Villain Factory and their distribution of a dangerous drug called Trigger, which creates hyper-powerful Instant Villains.

Within the narrative of Vigilantes, Aizawa serves as a key connecting link between the official hero society and the underground world of unlicensed vigilantes. His role frequently involves investigating the chaos caused by Trigger and clashing with or cooperating with those who operate outside the law. He initially confronts the brutal vigilante Knuckleduster, mistaking him for a villain due to his violent methods. After realizing Knuckleduster has no Quirk and is simply a determined citizen, Aizawa decides to collaborate with him to stop a common enemy, expressing gratitude while still warning him about the dangers of vigilantism. This establishes his pragmatic view that practicality and results can sometimes override strict legalism, even if he does not condone the lifestyle.

One of his most significant relationships in the story is with the young vigilante Koichi Haimawari, also known as The Crawler. Initially, Aizawa views Koichi with caution and tries to discourage him from his path. However, as they encounter each other more frequently, a begrudging respect develops. Aizawa saves Koichi from a powerful villain and later works alongside him during a city-wide chase, setting aside his duty to capture Koichi to instead prevent a civilian accident. This partnership shows that Aizawa is capable of trusting talented individuals who operate outside the system when the situation demands it. His relationship with fellow hero Nemuri Kayama, Midnight, is also crucial, as she is the one who submits a recommendation for him to become a teacher at U.A., a path he initially scoffs at. He also forms an unlikely alliance with petty criminals Jiro and Ichiro Hotta, using their shop as a makeshift base of operations and informant hub, much to their dismay.

Throughout Vigilantes, Aizawa shows notable development as a character. He starts as a young hero driven by past trauma and a desire to work alone, but through his interactions with vigilantes and the police, he begins to see the value in different perspectives. His conversations with Koichi cause him to reflect on his own past, including the incident with the kitten, and these moments help him move past his grief. By the end of his appearances in the prequel, he has accepted Midnight's recommendation, symbolizing his transition from a solely reactive underground fighter to a mentor who can shape the next generation of heroes in a more proactive way.

As a fighter, Eraserhead relies on his Quirk, Erasure, which allows him to nullify another person's Quirk simply by looking at them. This is an immensely powerful ability for a tactical fighter, as it can neutralize powerful opponents and force them to rely only on their physical strength. The activation of his Quirk causes his eyes to glow red and his hair to float, which can serve as a visual tell for his opponents. To combat this, he often wears specialized goggles that conceal his eye movements. The Quirk has limitations; it requires maintaining eye contact, and overuse leads to dry eyes. It is also ineffective against Mutant-type Quirks that are purely physical augmentations, as erasing the Quirk does not remove an already existing tail or extra limbs. Since Erasure provides no offensive capability on its own, Aizawa has become a master of close-quarters combat. He wields a carbon nanofiber binding cloth as a capturing weapon, which he can manipulate with precision to entangle and immobilize villains after canceling their abilities. His fighting style is highly agile and tactical, designed to end fights quickly and efficiently through strategy rather than brute force. This combination of a rare nullification Quirk and expert physical technique establishes him as a formidable hero who can handle situations that more flashy heroes cannot.

In summary, My Hero Academia: Vigilantes portrays Eraserhead as a pragmatic and effective underground hero whose past traumas and current investigations serve as a crucible, forging his philosophies on justice and mentoring. His interactions with vigilantes and fellow heroes in this prequel era directly lead to his eventual career as a teacher, making him a more rounded and understandable figure.