Hayato was born the son of Lavina and an unnamed mafia boss after being conceived during an affair between the two. Due to the mafia forbidding the birth of illegitimate children, Hayato was forbidden from seeing his mother throughout his childhod and was only able to see her three times a year. Lavina encouraged Hayato to play the piano, though she was unable to reveal that she was his mother.
In his youth, his older sister, Bianchi, would constantly make him eat her poison cooking whenever he had a piano recital, as his playing became terrible and distorted whenever affected by the by the poison, and was hailed by audiences as unique, modern, and abstract. As a result of his childhood trauma, he abandoned the piano despite his great talent and instead decided to join the mafia. Since he was considered to be a mere musician, no one wanted to accept him into their family, the fact that he was an oriental half-breed lessened his changes of finding a willing mafia family to take him in.
When Hayato was eight he discovered that Lavina was his mother and that she died in a car accident of unknown causes after driving off a cliff, which led to him running away from home.
In attempt to become stronger, he sought advice from Dr. Shamal, who at the time was working with his family. Hayato seemed to have looked up to Shamal, to the point that he may have copied his hairstyle, and even wanted to learn his assassination technique, the Trident Mosquito. Instead, Shamal taught him how to use sticks of dynamite, which sparked Hayato's fascination with explosives, eventually chosing them was his main weapon.
The choice of weapon lead Hayato to become known as a human explosive because he hides dynamite all over his body, giving him the nickname "Hurricane Bomb" or "Smoking Bomb".
As a teenager, Hayato was promised that he would be named the Tenth Boss of the Vongola if he were to defeat Tsunayoshi Sawada. He agrees, though he never wanted to be the boss to begin with and only wanted to see how worthy Tsuna was for the role.
Hayato first arrived in Japan to test Tsunayoshi Sawada's worthiness for becoming the heir to the Vongola Famiglia. However when Tsuna saved Hayato from his wayward Dynamite, he became fiercely loyal to Tsuna admiring him to the point of extremes.
During the Kokuyo Junior High Gang attacks, Hayato was ranked third strongest in Namimori Middle School and was attacked by Chikusa Kakimoto. During the fight, Chikusa realized Hayato was part of the Vongola and attacked Tsuna, forcing Hayato to dive in the way of the attack, taking the full brunt of the blow. Hayato was healed by Dr. Shamal and joined the others in the assault on the enemy base, though he became a liability, due to the side effects of his treatment. After several fights with various members of Mukuro's gang, Hayato faced off with Chikusa once again; however, his wounds allowed Ken Joshima to attack from behind. As he lay defeated, Hayato realized that Kyoya Hibari was being held captive in a cell behind him. He freed Kyoya, who fought Chikusa and Ken off. Hayato also gave Kyoya Shamal's cure for the Sakura-kura Virus. Soon after, Hayato was possessed by Mukuro Rokudo and forced to fight Tsuna, though he was eventually incapacitated by him.
A month later, Hayato (along with Takeshi and Tsuna) was defeated by the Varia henchman Squalo, who was chasing the Vongola messenger Basil. Hayato was later given the Storm Half Vongola Ring and tried to enlist Dr. Shamal to train him, who refused. However after causing much injury to himself while training alone, Shamal decided to train Hayato but not heal his wounds. Later in the week, Hayato and fellow guardians Takeshi and Ryohei Sasagawa defended Lambo from the Levi Lightning Squad. Hayato's training paid off just in time for his match and he invented the Rocket Bombs. In the Storm Ring Battle, Hayato fought the Varia genius Belphegor on the third floor of the school. Explosives were set up to detonate after fifteen minutes and despite figuring out each other’s techniques, the two ended up in a clumsy struggle for the Ring. However, Tsuna convinced Hayato not to risk his life, and, as a result, Belphegor was deemed the winner. Later, during the Sky Ring Battle, Hayato was given the cure to the poison by Kyoya and went on to defeat Levi A Than and cure Lambo and Ryohei. However, while trying to rescue Chrome Dokuro, both Hayato and Takeshi were caught in Mammon's illusion until they were saved by Ryohei. Later, the Vongola Guardians forced the remaining Varia to surrender.Shoichi Irie. Hayato and Tsuna were then attacked by Lal Mirch who later lead the two of them to the Vongola Base, meeting Future Takeshi on the way. After having the situation in the future explained to them they set out again with Takeshi to gather the other Guardians. The three soon found Adult I-Pin, Lambo and Haru Miura and Kyoko Sasagawa being attacked by Millefiore members, Nosaru and Tazaru. Takeshi then explained the use of Waves and Boxes to Hayato but was replaced with the past version of him. However Hayato managed to activate his Future self's Box Weapon to defeat Nosaru. They were later trained by Lal Mirch to use the weapons properly and Hayato went to follow up a lead at Namimori Shrine with Takeshi. They soon encountered Gamma who Hayato initially took on alone refusing to cooperate with Takeshi and were easily overpowered. But even when they worked together, Gamma far outmatched them both. During the battle, Hayato gave away that Tsuna is alive and was tortured but refused to reveal anything further. The Future Kyoya then showed up and saved them from Gamma. After recovering, Hayato then began training with his sister who gave him the Sistema C.A.I. his future self-worked on. However, the two worked badly together and he later escaped from her training. Despite this, he figured out how to use the Box Weapons by himself.
Merone Base Invasion
Hayato joined the others in the invasion of the Merone Base. Once the group was split due to the base moving Hayato and Ryohei encountered a recovered Gamma, who explained the layout of the base. Ryohei then tied Hayato up in order to prevent him rushing in but was defeated while protecting him from Gamma's attack, leaving Hayato with a rematch. Using his newfound power in Sistema C.A.I., he was able to keep Gamma at bay until the latter released his Nero Volpi, which overpowered Hayato. However thanks to the activation of Ryohei's Box Weapon, Uri transformed into its leopard form and came to his side. The fight ended in a huge explosion that caused the area to collapse burying the fighters in rubble. Hayato was later saved by the support group, and, later along with the other Vongola Guardians, was given a Vongola Box.
When returning to the past for the Arcobaleno Trials, Hayato was chosen along with Tsuna in Colonnello's Trial of Combat. At first the two struggled on their own, but once they worked together Tsuna was able to destroy Colonnello's gun and earn the first seal. Later Hayato and the other Guardians were assembled for Reborn's Trial of Leadership and were easily beaten by him. Shortly after, the Guardians joined forces with the other Arcobaleno, to defeat Verde and his army of Box Weapons.
Upon returning to the future Hayato was assigned by Dino to train Lambo and Ryohei (who had recently been replaced with his past self) to use Rings and Boxes. He was chosen as one of the participants in the Choice Battle but while defending Shoichi, Hayato had his Box Weapons rendered useless by Kikyo's Bellflowers. Hayato then attempted to pursue Kikyo on his Motorcycle but was easily disabled resulting in the Vongola's loss. While escaping Hayato used his storm missile to damage the teleport system and slow down the Six Funeral Wreaths.
After being sent back to the past for the Inheritance Succession, Fon became Hayato's tutor. Fon secretly aided G (the First Storm Guardian) in his trial, by distracting Hayato while G disguised himself as Hayato and performed the duties expected of a right-hand man (which included defending Tsuna from gangsters who had a grudge against Hayato). Upon revealing himself to Hayato and Tsuna, G stated that Hayato had failed his test as he proved unreliable as a right hand man. However once he saw the unbreakable trust between Tsuna and Hayato, he gave Hayato his Inheritance. Later, Hayato joined the others in rescuing the girls from Demon Spade. He and Tsuna were separated from the others and faced doppelgangers of them created by Spade. The illusion was then torn apart by Kyoya’s rampage.
Hayato was injured by the Six Funeral Wreaths while trying to save Uni. During the day of the final battle, he disobeyed Tsuna and worked with Lal Mirch and Gamma (who had recently joined the Vongola) to act as the first line of defence. They soon encountered Zakuro who after opening his Asura Box defeated Gamma and Lal. Using his Vongola Box Hayato was able to blow Zakuro's arm off, but before a conclusion could be reached Bluebell arrived and attacked. Hayato was then saved by the Varia and healed by Lussuria. When the final Funeral Wreath, Ghost, suddenly appeared Hayato, Ryohei, and Basil tried to use a combined Attack. However, the attack passed straight through and Hayato, (along with everyone else present) had his Dying Will Flames drained. The Vongola eventually defeated Byakuran and returned to the past.
Hayato soon took an interest in Shitt P!, one of the new transfer students from Simon Middle School. Believing her to be an U.M.A. (Unidentified Mysterious Animal) he began to obsessively keep track of her movements. He found great difficulty in understanding her and attempted to communicate with her by calling her by her preferred name "Shitoppi-chan".
Attack on Inheritance Ceremony
During the Inheritance Ceremony, Hayato was defeated by Enma Kozato along with the other Guardians, and their Vongola Rings were broken. However Talbot then appeared and repaired the Vongola Rings by combining them with the Animal Rings. With Hayato's resolve, his ring turned into the Buckle of the Storms Version X.
Ryohei vs. Kouyou
He then accompanies Tsuna, Ryohei and Lambo to the Simon's Holy Land and observes the battle between Ryohei and Kouyou Aoba.
Lambo vs. Large
Later on, when the Vongola members set up a camp for the night, Hayato and Tsuna fall into Large Ooyama's trap and become unwilling observers of the battle between him and Lambo. After Lambo activates his Vongola Gear, Hayato expresses his frustration that Lambo can't wear it because his hair is "full of useless crap". He then felt relief when Lambo used the 10 Year Bazooka and 15-year-old Lambo appeared, but quickly loses his cool when Lambo says that he lost all his memories of the past when he hit his head. Later when Lambo states that he has lost his will to fight, Hayato tries to get him fired up by telling him he needs to show his guts as a Vongola Guardian. Lambo then claims that comment caused him to lose his Dying Will. Later after Lambo rallies himself and defeats Large the Vindice appear and show the Vongola and Simon the second key. After witnessing the memory, Hayato exclaims that Cozzato chose Giotto to lead because the best person always leads the Vongola. When Large's questions say that Tsuna is not as capable as Enma, Hayato reminds Large that he lost to Tsuna's Guardian.
Vs. Shitt P!
Later Hayato, Tsuna and Lambo walk into a ghost town on the island. There they encounter Shitt P! who doesn't respond until Hayato decides to use her nickname. The battle begins as Hayato reveals his Vongola Gear's Cambio forma: dynamite. The battle between Hayato and Shitt P! begins. Shitt explains the rules of the battle are to pop their opponent's 2 Fiamma Balloons that stay 1 meter to them. Hayato accepts that he pops the balloons rather than hitting a girl, even if she was an UMA. Shitt P! says that the entire town is the field and that the duel has to be settled in 5 minutes or else it will be a tie. Hayato tell Tsuna to take cover somewhere far away. Hayato uses Rocket Bomb Version X, a stronger, faster version of Rocket Bomb which also includes homing abilities. Shitt P! uses her ring weapon, in the shape of 8 scorpion tails, and uses it to melt the bombs.
Shitt P! throws away her weapon and latches on to Hayato in a hugging pose and comments on Hayato's warmth and naivety. Shitt P! explains her belief that living with others is a sign of weakness. She states that Hayato lives for the sake of Tsuna because he has no confidence in himself and that if Tsuna died, Hayato would no longer have a reason to live. She continues on, mentioning that she loves her fellow family members, and she loves Hayato as well because he is cute, but ultimately loves herself more than anything and pops 1 of Hayato's balloons. When the ground around Hayato turns to mud, Shitt P!'s reveals her Swamp Flame, able to ferment any inorganic substance to create a bottomless swamp.
Hayato starts to sink so he uses a new explosive, Air Bomb, that manages to propel him out of the swamp. Shitt P! explains why Tsuna is a loser and tells Hayato that if Tsuna wasn't the Vongola Boss, he would just think of him as an ordinary loser. Hayato shockingly looks past all of Tsuna's faults and evens the score by sending Shitt P! flying while popping 1 of her balloons. He explains how he knows Tsuna is a loser, but his faults have nothing to do with being the Vongola Boss, whom Hayato respects greatly. Hayato pulls out more dynamite and prepares to attack again. This time he uses the dynamite to lure Uri with catnip. Uri uses her true power as a weapon and explodes thus destroying Shitt P!'s other fiamma balloon. With this, Shitt P! is defeated and Vindice come to take her.
Enma, who was overwhelmed by anger, ambushed them, leading to a brief fight between him and Tsuna until Enma reveals that Tsuna's father, Iemitsu, has killed his parents and little sister. The stunned Tsuna was about to be killed by Enma, but was saved when Enma's ring went out of control, forcing him to retreat. Hayato became concern of Tsuna who became unresponsive out of depression after hearing Enma's story. Hayato questioned the credibility in Enma's story, but Tsuna stated he was able to feel Enma's anger and hatred when they exchanged fists.
Kyoya Hibari vs. Adelheid Suzuki
The group encountered Adelheid, who wants to fight against Tsuna, but Hayato steps in despite being tired from his battle with Shitt P!. Unfortunately, Reborn tells him to forget about it since he had used up all of his Flames against Shitt P!. Kyoya then arrived and took the challenge instead, prompting them to watch the battle. Once Adelheid was defeated, Julie Kato reveals himself as the first Vongola Mist Guardian, Demon Spade, and the one responsible for setting up the war between Vongola and Simon. Hayato questioned Demon Spade's existence, wondering if he is a ghost, something that Reborn also questioned. Kaoru then attacks Demon as the latter reveals that the former was the one who attacked Takeshi. When Demon whom Kaoru attacked is revealed to be an illusion and Kaoru got cornered, Tsuna contemplated to do something, but Hayato angrily reminds Kaoru's role in Takeshi's attack.
Before Kaoru can be killed, Takeshi arrived and saved him, shocking everyone, but Tsuna and Hayato were nevertheless relieved that he has recovered. After a clash between Takeshi and Demon that forced Demon to retreat, Hayato and the others decided to take a rest and exchanged what they have known so far.
Tsuna vs. Enma
Arriving at underground castle, Hayato watches the fight between Tsuna and Enma who was overwhelmed by his hatred and anger and the awakened Simon Ring. Tsuna succeeds in snapping Enma from his condition and saves him from the out control power of Simon Ring. Emma apologize for what he and his Family has done, but Hayato tells him that Demon is the one at fault. He then watches the memory of Giotto and Cozzato before going to where Demon is.
Tsuna and Enma vs. Demon
The group to encounter Mukuro after he supposedly defeated Daemon, much to their shock. Mukuro then allows Chrome to be in control again, and Hayato commented that Chrome is strange for worrying about Mukuro than herself despite the fact she was the one who got kidnapped. Their cheery moments were interrupted by Mukuro, who possessed the Mist Owl because he couldn't return to his own body, deducing that Demon actually lost on purpose so he could possess his empty body. The Vindicare then appears and reveals that Demon has succeeded in prison break and tells them that the Vongola must handle Demon themselves since Demon is still a member of Vongola Family. Tsuna agrees in exchange that the imprisoned Ryohei and Simon Guardians being released.
Shortly after, Demon appears and sends Hayato, who pushed Tsuna out of the way, together with Lambo, Takeshi, and Julie into his illusionary world, where they watched the battle. Following Demon Spade's defeat, he is released and receives the last key with the others. After the events end, he is last seen on the way to school while running away from Shitt P!, who chases him relentlessly after developed an interest in him.
Hayato appears with Tsuna at Chrome's welcoming party because Chrome was kicked out of Kokuyo Land. In the miming competition, he mimed Takeshi as a baseball player, but was kicked off by Reborn due to having a performance with a "malicious intent".
1st Day of Representative Battle of the Rainbow
Later, Hayato appears the day before Tsuna's father Iemitsu arrives in Namimori, commenting to Tsuna that he was excited to be fighting for Reborn to allow him to revert to his adult form. On the first day of the Representative Battle of the Rainbow, Hayato happily heads to school with Tsuna. However, when the battle suddenly starts, he, Takeshi, and Ryohei group up and try to find Tsuna. When an explosion occurs because of Tsuna being punched through the ceiling by Iemitsu (though they don't know the cause), they run towards the explosion; however, they are barred by Kyoya and Fon, the former appearing to be Fon's representative, who states that he would bite all three of them to death. After Ryohei's watch was broken, Hayato and Takeshi worked together to create a trap for Kyoya by using the Rain Flame's Tranquility characteristic to immobilize him and were able to escape.
2nd Day of Representative Battle of the Rainbow
After the defeat at the hands of Iemitsu, Tsuna is reluctant to go home. Hearing this, Takeshi invited Tsuna to stay over at his house, which he agrees along with Hayato. The next day after school, the three of them goes to Uni's place and was happy to see her alive. Along with the other members of Team Uni, they discuss the result of the first day battle and Team Verde's surprising battle power. Not long, however, the wrist watches announced the second battle within 30 minutes time limit. They were ambushed by Team Verde and in the middle of the battle, Team Colonnello broke the alliance with Team Reborn since Tsuna refused to leave Team Uni. After Byakuran's boss watch was destroyed and Mukuro retreated, Hayato and Takeshi goes to where Tsuna and Iemitsu is using bicycle with Hayato using his dynamites to speed up, only to arrive when the battle time is over.
They are surprised when Checker Face announced Team Bermuda's participance in the representative battle and immediately visit the injured Team Skull who had their watches stolen by the Vindice. After visiting the hospital, they were ambushed by a Vindice outside battle time. They able to hold up enough until Reborn arrives.
3rd Day of Representative Battle of the Rainbow
Shortly after fighting the Vindice and when they're about to sleep after feeling exhausted from the battle, the watches announces the start of the third battle. They immediately went to the park and met Mukuro there, followed by his teammates and was about to begin the battle. However, three Vindice appears, leaving both team no choice but to form temporary alliance to defeat them. Hayato, Takeshi, Ken, and Chikusa are easily defeated once the Vindice fight in their true forms, leaving only Tsuna and Mukuro to fight them. After the third battle was over, Hayato and Takeshi are taken to the hospital.
Tsuna explained everything that he heard from his conversation with Bermuda and also his plan to save Reborn and the other Arcobaleno. After Tsuna left, both comments that it's the first time that they ever saw Tsuna so calm despite how dangerous the situation at hand. This made Hayato worried since he feels that Tsuna is quickly leaving him behind. In the afternoon, he, and everyone Tsuna has talked to gathered at Tsuna's house where Tsuna explains his entire plan and information and asks them to fight together.
4th Day of Representative Battle of the Rainbow
He, Takeshi, Chrome, and Fran are chosen to participate in the fourth day battle against Team Bermuda. They are in charge on holding up Big Pino and Small Gia who are after Team Verde. Hayato's Sistema C.A.I was destroyed by Big Pino's cannon when he was protecting himself, Takeshi, Chrome, and Fran while hiding inside illusions. Tsuna, Enma, and Basil arrives in time before Big Pino's attack could kill them. After exchanging weapons, Small Gia and Big Pino easily corners Tsuna, and destroying Basil's wrist watch. Before Small Gia able to finish Tsuna, Takeshi restrains Small Gia from behind while Hayato distracts Big Pino and with Enma's help, able to reach Big Pino's arm near enough to destroy his watch, stating that they need his permission first if they wish to fight Vongola tenth. While Enma was stopping the Vindice's movements, Hayato tells Chrome to use something that she knew. Understanding this, Chrome aid Fran to protect Hayato and Takeshi with Verde's device by covering their whole body as Hayato's dynamites explode. From the smoke, Hayato and Takeshi emerges, happily said that the Vindice's watches have been destroyed and declares not to underestimate the Vongola.
In the final chapter, he and the other Guardians gather outside Tsuna's house to celebrate.