Eizo Tanuma/Synopsis

History
At least seven years ago, the hero O'Clock requested the help of the Police Force to help him in his operation to investigate the Underground Masquerade, a clandestine fighting event that took place in Osaka, where Quirk-upper drugs were also trafficked, and there were also knowledge of some fighters missing.

Eizo and a small Police Force surrounded the building where the illegal event took place, and the detective asked if he wanted mim and his boys to raid the place, but the Pro Hero replied that there is no point in that. O'Clock explained that his target is not the event itself, but rather the drug trafficking that occurred there. He had knowledge that the fighters are using quirk-boosting drugs and he was heading there to secure some evidence.

Eizo realized that the police were there to secure an escape route for him after he did what he needs to do there. O'Clock asked if he has any more questions, and the detective said only one: "What’s with the Five O'Clock Shadow?" The hero just smiles and took off saying he has to look the part. Resigned for not getting an answer, Eizo relayed the new orders.

O'Clock infiltrated the event and obtained the evidences he needed, but the situation got complicated when All For One, the person behind the Underground Masquerade, decided to steal some Quirks and released a gas that made all those who breathe it lose control of their Quirks and go on a rampage. Detective Eizo was waiting outside the Underground facility, telling the rest of the backup to standby until O'Clock returns, when he noticed the gas from inside starting to make its way out the door, wondering what's going on down there.

In order to escape, O'Clock teamed up with two fighters, The Rapper and Tiger Bunny, and devised a plan to get out of there, including a plan B in case they had to face an unexpected threat. They put the plan into motion, and when they almost reached the exit, a mysterious hooded individual sent by AFO appeared to stop them.

Since they could barely handle him, O'Clock ordered execute Plan B: while O'Clock and The Rapper hold the Hooded Man, Tigger Bunny busted her way out of the building, as the backup police look on in confusion. She called for Detective Eizo, giving him the message from O'Clock, to call All Might.

Vigilantes Beginnings Arc
Along with his partner Naomasa, Eizo is dedicated to investigate the "Villain Out-Break Incidents", a series of events related to the Trigger distribution that has caused several Instant Villains outbreaks in Naruhata. After interrogating one of the dealers and several consumers, Eizo deduces that there must be an organization behind the drug distribution through the streets.

Since Trigger is not a cheap sustance, he is confused as to why the dealers would hand Trigger out for free. Based on their actions, he believes that the mysterious organization couldn't care less about financial side, they no seek economic benefits but a goal on a much greater scale, although still he does not know what it is.

After interogating one of the former Instant Villains, Teruo Unagisawa, the police decide to let him go. Eizo advises Naoamsa to follow him, in case he is lucky, and leads him to those who provided him with the Trigger. Unfortunately, the villain Kuin Hachisuka causes a sudden out break of Instant Villains by injecting Trigger into several innocent people, and takes advantage of the chaos to take Teruo away, while the heroes suppress the outbreak.

Days later, due this sudden villains outbreak, Eizo organizes a meeting with several of the heroes who helped to stop this. However, due All Might’s tendency to run off in a hurry to carry out his activities as a hero it is impossible to schedule a meeting that would work for him, so he sends his partner Naomasa to the Might Tower to know All Might’s opinion. Just in case he is not able to meet with All Might, Eizo recommends Naomasa to meet up with Toshinori Yagi, from All Might's Secondary Clerical Office.

At the police station, several heroes arrive to participate in the meeting. Eizo explain to them that the instant Villains were normal citizens who have been forcibly injected with the drug Trigger. The hero Endeavor understands that what he means is that they are more victims than villains, so he suggests that the heroes pay their medical bills, and make sure the media they tread lightly when reporting this.

But Eizo not only wants that but also that the heroes do not rush when it comes to facing an Instant Villain, in case it is another innocent person. Endeavor considers his request ridiculous and argues that any public use of a Quirk is illegal and that they should treat them as villains and let the police investigate the backgrounds of future captures. Endeavor get more enraged with Eizo's continued hesitation and tells him to go ask for All Might's opinion.

At that moment, Naomasa arrives with a statement from All Might declaring that he will assess the situation at the scene more carefully because as heroes they should try to keep the innocent citizens out of harm's way. This leaves Endeavor quite literally burning with rage once the meeting ends.

Sky Egg Arc
Weeks later, Naomasa leads a police operation to dismantle Trigger's main illegal import route. The route is dismantled, but the villain responsible for it manages to escape after destroying all the evidence, and leaving the entire police force badly wounded with his quirk. When analyzing the recordings of the security cameras, Eizo suggests that the villain must have an acceleration type Quirk. This gives Naomasa the idea to investigate an old hero known as O'clock, who had a similar quirk and thinks he may have some kind of connection with the villain.

Long after this, Eizo secretly meets with Knuckleduster, who was the hero O'Clock before his quirk was taken from him. Knuckleduster gives him important data about the importers of Trigger in Japan, which makes the police investigation progress remarkably. For his part, Eizo informs him that the police had an encounter with a villain with a Quirk similar to O'Clock. Knuckleduster assures him that he does not know who this villain is, but he advises Eizo that if the police encounters that criminal again, they should not hesitate and shoot to kill.

Days later, after the Sky Egg Bombing, Eizo and the police are investigating a dilapidated building near where the incident occurred. They ignore what could happen, but when obtaining clues and blood samples, they deduce that at least two people were involved in the collapse of the building.

Naomasa informs him about the results of the blood samples recovered from the place. One corresponds to the former hero O’Clock, while the results of the second sample came back inconclusive as they found his blood type and DNA was mixed with a number of other chemicals. Eizo, who lead the Trigger investigation and knows the truth about Knuckleduster, questions if "the thing they're chasing" is even human.

Final Performance Arc
Year later, there is an incident in Naruhata caused by Pop Step, who had been kidnapped and transformed into Bee Pop by Number 6, the villain responsible for the incidents related to the Trigger years ago. Fortunately, she was stopped by Koichi and his allies Soga, Moyuru and Rapt, who rushed her to a hospital, where the parasite bee that controlled her was removed.

After this, Koichi disappears and Soga, Moyuru and Rapt are questioned by Naomasa to tell them everything about Koichi and Kazuho, due to the atypical, but neither says anything about it, although Soga asks him to bring in Detective Eizo. Naomasa complies.

Eizo enters the interrogation room, and tells Soga that he knows about his involvement with Trigger a few years back. Eizo notes that Soga has a different feel to him compared to back then, reminding him of a certain ally, grabbing Soga's attention, and brings up an analogy about socks that their common ally told him about. Eizo chuckles at the analogy, and later lets him and his friends go.

Naomasa is bewildered that Eizo let the thugs go, while the latter tells that it's better to pit troublesome people like them against actual villains. As the three former villains walk away, they see Naomasa and Eizo looking down at them, but they continue walking away. Naomasa tells that he will continue watching them, as well as The Crawler, while Eizo nonchalantly pokes fun at his uptightness.

Current arc
Detective Eizo finds officer Sansa working at his desk. Eizo asks about Naomasa, and he replies that he went back to the hospital. Sansa also informs him that he was able to uncover some information about the hospital as he had requested him and it turns out that two years prior to the secure ward was constructed, the hospital had been receiving generous donations by a man who goes by Takeshi Kuroiwa. Sansa goes on and says that benefactor holds several positions on several companies and non-profits, but there aren't many public details about him.

Eizo knows who he is talking about and thanks Sansa for the information. The officer asks him about the envelope he brings with him. Eizo tells that it is an case in which he participated years ago, and that there is something he wants to check. The case is that of the Underground Masquerade.