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.

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.

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.