Aki Hayakawa/History

Intro arc
When Hayakawa is first introduced, Makima orders Denji to work with him. While they are patrolling the city, Hayakawa becomes annoyed with Denji asking whether Makima has a boyfriend and pulls him into an alley to beat him up, as a warning that if he doesn't take Devil Hunting seriously he could end up dead. Denji then fights back, kicking him in the groin multiple times and stating that he would risk his life to continue living the way Makima has shown him. Once they get back to the Devil Hunters Headquarters, Makima assigns Denji to Hayakawa's squad, while also arranging the two to live together.

Bat Devil arc
Aki drives Makima from her meeting with her superiors. In the car, Makima explains to Aki that she is interested in Denji because of his ability to transform into the Chainsaw Devi. As Denji is impaled by the Leech Devil's tongue, Aki arrives at the battle. He summons the Fox Devil to devour the Leech Devil and save Denji. After taking Denji to the hospital, Aki explains that he was able to summon the Fox Devil as he made a contract with it, and that he allowed the Fox Devil to consume a part of his skin in exchange. Aki expresses his disdain for devils to Denji, claiming that they cannot be trusted. Aki informs Denji that he knows that Power tried to kill him, which Denji denies. Aki then threatens to report Power, along with Denji for covering for her, and have them executed if they do not obey his orders as their superior.

Makima informs Aki that Power is going to be moving into Aki's apartment, to his dismay. Both Aki and Denji yell at Power for never bathing or flushing the toilet.

Eternity Devil arc
Aki and Himeno are seen extracting a part of the Gun Devil's flesh from the dead body of the Fish Devil.

After receiving a devil extermination request from the Bureau, Aki, Denji, and Power head to Hotel Morin along with Himeno's group. All of them proceed to explore the location, floor by floor. They come across a devil in the shape of a walking head, which Himeno and Power handled swiftly.

They continued on until the eighth floor, where Hirokazu stated that they were supposed to be at the ninth. Upon further investigation, Aki soon came into the realization that they are trapped in the eighth floor, possibly due to a devil's ability. Aki continues to search for the devil, eventually returning to the room where the group is gathered, informing them that the devil they killed earlier is constantly increasing in size. The devil forms a contract, offering their freedom at the cost of Denji's life.

As the situation becomes more and more desperate, the rising fear of Kobeni and Hirokazu feeds the devil's growth. The devil then chases after the group, tilting the space they're trapped in and transforming the corridor into a tunnel that leads into his mouth. Aki, ready to engage in combat with his sword, ask for permission from Himeno, who restrains him and apologizes to Denji, as Hirokazu and Kobeni prepares to kill him. Aki breaks free and shields Denji from the stab, receiving an injury in the process. Denji decides to take matters into his own hand and jumps down, telling Aki that they'll call it even if they manage to survive.

After three days, the Eternity Devil surrenders, exposing its heart so Denji can end its pain. After its defeat, Aki and the others make their way out of the hotel. Sometime after the incident, Aki and Himeno recovers a part of the Gun Devil from the body of the Eternity Devil. Himeno suggests throwing a party for the entire 4th Division in celebration of their successful mission. Aki suggests doing it before the next week, as Makima will head to Kyoto then, and Aki wouldn't go without her. At the party, Aki asks Makima for more information on Denji, with her stating that she will tell him if he is able to outdrink her, which he and Himeno fail to do.

Katana Man arc
Aki, Denji, Power, and Himeno eat out at a ramen shop. A man starts talking to Denji, and Aki asks if Denji knows him. The man suddenly shoots Denji in the head and shoots at everyone else. Aki dodges the shot and summons the Fox Devil, who consumes the man. Aki then look around and sees an unconscious Denji and Himeno, who has been shot. The Fox Devil tells Aki that he put something monstrous in her mouth, which is nor a human, nor a devil. The man then transformed similarly as Denji, but with swords instead of chainsaws, opens the Fox Devil head and comes out. Aki pulls out his sword, a large nail, and tells Power to stop Himeno's bleeding. As the Katana Man and Aki fight, Aki pierces him three times with his sword, then asks the Curse Devil to kill him. The Curse Devil appears, grabbing Katana Man and killing him.

Aki wonders how he had a gun, as only police officers and Devil Hunters can have guns in Japan. Before Aki can focus on Himeno's safety, a woman suddenly shows up and deduces Aki's ability with the Curse Devil, then revives Katana Man. She tells him to go and kill Aki, so Katana Man takes a stance and, in an instant, moves past Aki and slashes him. While Aki falls to his knees, he is still alive, so the woman tells Katana Man to finish him off. Himeno decides to give up all of herself to the Ghost Devil to try defeating Katana Man and save Aki, but after the Snake Devil is summoned and consumes it, her entire body vanishes, leaving only her clothes behind. Aki is left to take in what happened, left in a state of shock and silence.

Aki reawakens in a hospital bed and sees Denji and Power sitting next to him and reading manga. When Aki asks who is still alive, Denji reveals that Kobeni and Madoka survived, but Madoka quit. After Denji and Power leave, Aki takes his sword and asks Curse Devil how many years he has left, and Curse Devil tells him that he has 2 years left. Aki takes a cigarette, and as he tries to use the lighter, he remembers Himeno and begins to break down in tears. Shortly after, Michiko and Yutaro visit Aki and introduce themselves as career coaches who came per Makima's request. Because of his loss of the Fox Devil due to getting on its bad side and his limited use of the Curse Devil's sword, Aki is told that he must make a contract with a stronger devil if he plans on continuing devil hunting. Aki states that the one who killed his family and the one who killed his friends are still alive, so he can't quit. A woman then enters, surprising Aki.

The day after, Michiko and Yutaro take Aki to an underground special facility, where Public Safety locks up devils they capture. Yutaro then recalls seeing the girl that entered Aki's room after them and wonders if he is his girlfriend. Aki explains it was Himeno's sister, who gave him Himeno's letters to her family. Reading them, Aki saw that Himeno was thinking about how to make Aki quit. They take him to the cell holding the Future Devil. They warn Aki that two people have a contract with him; one gave half of their life span, while the other gave it their eyes, senses of taste, and smell. Aki then enters the cell and faces the Future Devil. The Future Devil tells Aki to stick his head into his gut, which he follows. After a few moments, the Future Devil laughs and states he wants to live in his right eye, and that way, he will grant him his power. Future Devil explains it is because Aki will die in the worst way possible. Before he can say anything else, Aki stops the Future Devil, as he is uninterested, and tells him to get in his eye.

While driving him back to HQ, Yutaro criticizes Aki's goal of killing the Gun Devil. Aki doesn't care what they say and tells them to sit back and watch, and if he ends up dead, they can laugh at him. After dropping Aki off, Yutaro throws him a cola and states that even though Aki pisses him off, he is rooting for him and warns him to be careful.

Gun Devil arc
Aki is killed by the Gun Devil as it advances towards Makima. The Gun Devil then possesses his corpse and they become a Gun Fiend. Within this new form, he returns to his residence in order to ambush Denji. Within his mind, Aki hallucinates the experience as if he is having a snowball fight with Denji. Using his left arm, which became a gun, he fires at Denji, destroying his apartment and tearing Denji in half. He and Denji then engage in combat, which causes many building to be destroyed due to his attacks, injuring and killing bystanders in the process. In his vision, Aki sees Denji crying, causing him to give up, thus giving Denji the opportunity to kill him. Aki then turns around to see that his brother has returned with the gloves, both of them proceeding to play catch together in the afterlife.

It's later revealed that in his will he left half of his money with Denji and the other half with Himeno's family.