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Koyo Aoba is a member of the Simon Famiglia. He previously attended Simon Middle School, but he and his family transferred to Namimori after an earthquake hit their area. They chose Namimori because it is relatively earthquake free and they are invited to the Vongola Decimo's Inheritance Ceremony. Koyo uses boxing gloves as his weapon, just like Ryohei. Both he and Ryohei also have the same temper, as seen when they quickly get angry and neither of them want to stop the fight. He is sensitive over his contradictory name (Aoba meaning "green leaves," and Koyo meaning "red leaves"). He is firm in his belief that, no matter what his first and last name mean, they are still the same leaves.
Koyo's regular attire seems to consist of his school uniform, and despite his appearance, it belies his brash personality similar to Ryohei. Furthermore, it hides his incredibly fit body, which usually appears slender with his top on. He commonly wears small-framed glasses that are pitch black when looked through but seems like regular glasses to anyone looking at him, and possesses green hair as well as red eyes which seems to be a trait shared amongst the other members of Simon. His bespectacled appearance gives off the impression that he is a bright individual; however, it only belies his actual intelligence.
Koyo and Ryohei Sasagawa share the same love of boxing. Koyo even has self-proclaimed himself as "the Hope of the Boxing World." Despite his love of boxing, his hate for Vongola exceeds and takes precedence over it. In fact, he has stated that he has no boxer pride as his fists are a way to live. This contradicts Ryohei, who has incredible respect for boxing and all of his aspects and fights for is friends. In combat he seems to favor anything but a fair fight, preferring to fight allowing everything that is forbidden in sanctioned boxing including "throwing sand in the eyes, kneeing, and any other form of foul play" Another similarity between the two is that both have some kind of catchphrase. Ryohei refers to many instances as "Extreme" while Koyo tends to end his sentences with "In the end."
He seems to be the most brash member of the transfer students and also seems to portray a more colder side. He easily becomes enraged at even the slightest negative comment towards himself or his Famiglia. Koyo showed disapproval of his Famiglia being considered beneath Vongola when Gokudera made said comment, which could have escalated if it were not for Adelheid intervening. Still, Koyo seems to have bitter relationships with his Famiglia members. One such example is Julie, who he considers lazy when he refuses to partake in his duties. He is not above making fun of Enma, who is the boss of the Famiglia. He seems to be more respectful to women, or at least finds them more tolerable as shown within his interactions with Tsuna's mother.
He first appears with Ryohei when they have difficulty answering questions in class. He is later seen on the school's rooftop with the other transfer students and Tsuna's Guardians, watching Hibari fight Adelheid. Koyo even encourages Adelheid. to "defeat the Nami student." He appears to be surprised that Hibari did not waver at Adelheid's beautiful underwear-showing attack, and that he was able to push Adelheid across the rooftop. Tsuna (who is pushed by Reborn) stops the fight by coming between Hibari's and Adelheid's attack. Reborn says that the Simon are the Vongola's guests for the Inheritance Ceremony, surprising Koyo with his ability to speak fluently. Reborn even states that the Simon Famiglia has been their ally even before the time of Primo, but he did not know of them because they have become weaker over time. Koyo is riled up that Reborn speaks the truth so plainly and asks if Reborn has never heard of sugar-coating his words. Later, Koyo is seen with other Simon Famiglia Members and Vongola Guardians at Nammy's restaurant, discussing how to protect Tsuna in each area of Namimori. Gokudera creates a plan which separates everyone into groups. Koyo is aggressively against working under the Vongola, since he believes that the Simon Famiglia should be treated as equals, but he drops it after Adelheid quiets him. Before arrangements could be made, Adelheid decides that she needs to consult the other members of the family. At the Simon Famiglia hideout, when Adelheid proposes protecting the Vongola Famiglia, Julie states that he is not feeling well, so he will not help. At this, Koyo starts another argument, complaining about how he will only chase women and gamble instead. Koyo also starts to tease Enma for being weak and not trying to win a fight. Adelheid asks Enma to show her the bulky pendant that he has hidden under his shirt, saying that the pendant is proof that he has inherited the title as the next Boss of the Simon Famiglia.
The Simon Famiglia decides to guard Tsuna, with the first day being Koyo's turn. Due to their competitive natures, Koyo and Ryohei compete to see who is a better bodyguard. As they could not physically fight, Reborn makes them compete to solve a set of problems the quickest. However, the problems are printed on plates made of extra hard ceramic. Reborn states that the competition would have to be postponed if the plates were to break. The problems are too difficult for either of them to answer, so they try to break the plates. Alone, they could not destroy them. They realize that they must work together, so Koyo holds the plate while Ryohei breaks it by using Maximum Cannon. Reborn notes that Koyo is able to endure the punch without suffering any injuries and concludes that the Simon Family is not weak at all.
Koyo and the rest of Simon Famiglia are later seen at the hospital with the others about Yamamoto's condition after he was attacked in the baseball locker room by Kaoru Mizuno. Koyo also tells them that they had no leads, but was interrupted by Reborn, who said that he had found a lead. This news surprised the Shimon Famiglia, but Reborn said that he couldn't share it with the Simon Famiglia since it involved "secret information about the Vongola." This news does seem to spark an interest to the Simon Famiglia, but Reborn says that the Vongola needed some privacy. He does, however, give them a warning that they also might be attacked. Koyo is later seen at the ceremony with the rest of the Simon Famiglia. They all were surprised to see Yamamoto well. He is later seen amongst the crowds during the ceremony waiting along with the rest of the Simon Famiglia, when Tsuna was to receive the Vongola Sin from the Ninth. After an explosion, the sin was broken open, supposedly by Enma. The ninth revealed that the Sin in the ceremony was merely a fake. He proclaimed that the real one was in a safe that was impossible to penetrate. Suddenly, the safe is broken open, revealing Koyo and Enma with rest of the Simon. Enma states that they came to the ceremony to take back "sin" for the Simon Famiglia, and get revenge on the "Vongola". Enma later explains the story about Sin and the first boss of the Simon Family, Cozart Shimon who was believed to be betrayed by the Primo. But Tsuna refuse to believe this and proclaims "I don't deny that the blood of the Vongola runs within me. Neither do I have the ability to really find out what happened between Vongola and Shimon. I can't prove that it didn't happen. But there is one thing I can swear on my grave. Vongola Primo is man who would never do such a thing!". Koyo replies if their "mocking us?, you speak as if you've met him before". The Simon are angered by Tsuna words and release their Simon Rings, which was said to be a power that could rival the 'Seven Flames of the Sky' - the 'Seven Flames of the Earth'.
Enma quickly defeats Tsuna's Guardians alone, and then overpowers Tsuna. The Simon then leave and take Chrome with them to their "Holy Home", as Adelheid stated. After Tsuna and his remaining Guardians have recovered and upgraded their Vongola Rings, they head off to the Simon Base to stop them and to get Chrome back. The Vendice Guardians too arrive, and claimed that since the First Simon boss, and Vongola Primo were friends, the two made an agreement to never attack one another, or the losing family would have to go to prison. The Vendice state that since this is the second time this happened, the two Families would have to fight one another, and the loser would go to prison forever.
After Tsuna and his friends are walking through the forest looking for the area where the Simon are hiding. They arrive to a clear field in the middle of the jungle. Suddenly leaves start propelling themselves out of the trees, hurting Tsuna and his company. It is here that Koyo reveals himself and tells them that in that very area, the blood of Cozart Shimon was spilled by Primo, and proceeds to tell them that it will be Vongola blood that will now be spilled. Ryohei responds that he will take him on one on one. So Koyo conjures up a field of thorns using the flames from his ring to simulate a boxing arena.
The battle begins between Ryohei and Aoba. Aoba uses a move which envelops Ryohei in leaves and cuts him continuously. This move traps Ryohei and allows Aoba to connect several punches before Ryohei finally apologizes to Aoba for keeping him waiting. As the fight continues Koyo believes he is about to make the final punch when Ryohei moves in, however he sees it is going to be a counter and guards from it when Ryohei unleashes his new attack Sunshine Counter which blasts a huge burst of flames that destroys a huge majority of the area. Ryohei then explains the more damage he takes the more power his flames get. Koyo continues his barrage as Ryohei dodges only enough to get non-fatal injuries. Koyo believes Ryohei isn't dodging well enough until two flames light and a new attack called Sunshine Uppercut is directed at him and knocks his glasses off.Soon after, it is revealed by Lambo that the glasses basically made Koyo blind, because his eyes had the ability to see through things and his glasses were made thick enough to control his power. Koyo then tells Ryohei that before Ryohei dies, he should regret making Koyo have to use his full power. His eyes then start to light up.
Koyo then starts to project a large quantity of Forest flames from himself while Ryohei rushes towards him. The fight continues until it is revealed that Koyo has Killer Point Vision. Koyo hits one of Ryohei's Killer Points which makes Ryohei start screaming out in pain. Koyo reveals that his glasses are a sort of self handicap that keeps his Killer Point Vision under control. Koyo then proceeds to keep hitting Ryohei's Killer Points until Ryohei's bangle reaches 60% power. Ryohei then becomes ensnared by some barbed vines that Koyo has produced. After telling Koyo that he won't give up Ryohei breaks free of the vines and tries to rush Koyo. Koyo tries to attack Ryohei but his eyesight starts to disintergrate. Ryohei then hits Koyo point blank with a 60% power Sunshine Uppercut.
Koyo landed against the barbed ring and it appeared as if Ryohei won. Reborn stated that although Koyo has his Killer Point Vision, he can not use that technique for long and is forced to wear those glasses. As if all seemed over, Koyo screamed saying that it was not over yet and he won't be brought down by such a weak punch. Shocked by this predicament, Tsuna and the others wondered if he was unscathed by Ryohei's 60% power Sunshine Uppercut despite the fact that there is a hole in his abodmen. Reborn stated that this fight is a fight he can not afford to lose, so that's why he isn't knocked out yet. Ryohei finally realizes he needs to be at full power to defeat him and the two boxers begin to fight again. The question is whether Koyo can defeat Ryohei before his eyes overheat, or will Ryohei defeat Koyo with his full power attack. Quickly they charge at each other and it seems if Koyo is overpowering Ryohei. After all of Koyo's attacks, Ryohei barely seems to be able to stand properly, making sure he doesn't fall on his knees. Ryohei mocks Koyo and asks if Koyo is scared and wondering if he is done with his weak punches. Koyo, enraged charges at Ryohei and lands a right punch on Ryohei. However, Ryohei takes the opportunity and grabs Koyo saying he's gotten Koyo trapped. Finally, Ryohei has his Vongola Gear of the Sun at full charge. Soon after Koyo is hit with Ryohei's Maximum Sunshine Counter. However, it is revealed that he is still standing after the attack. After screaming out each other's names Koyo and Ryohei rush at each other and both pass out as they try to land a punch on each other. The Vindice then appear and declare that the match was a draw as they pull both Ryohei and Koyo through a portal.
At the end of Daemon and Tsuna's battle, he and the guardians of both Famiglias are released from the Vendicare Prison.
Koyo is seen voicing his dissent at allowing Skull to stay, misinterpreting Skull's request, scoffing that Skull was bragging about being a "pretty-boy hottie" and being over "seven heads tall".
Later, when Adelheid stumbles upon Enma and Skull after nearly getting killed by Team Mammon on the first day of the Representative Battle of the Rainbow, Adelheid thanks Skull for saving Enma and agrees on behalf of the other Simon Guardians to become part of his team in the Representative Battle. Thus, Koyo joins Team Skull.
On the second day, no one challenges Team Skull until just before the battle ends, whence a group of Vindice that eventually become known as Team Bermuda appear and defeat all of Team Skull in an instant, stealing their Rainbow Wristwatches to enter the battle as well. Team Skull is mortally wounded, but still alive at this point.
Weapons and Abilities
Like Ryohei, Koyo has a great love of boxing, though he see's it as a means to live rather than just for sport. As such he is not against using foul means to win. He appears cocky when in battle; though, with the abilities he has shown, he has reason to be. On the battlefield, he has incredibly quick reflexes, amazing speed, lightning-fast jabs, and devastatingly powerful punches that can easily break bones. As well as this he has incredible endurance, even after receiving Ryohei's attack at 60% power he was able to fight normally and was still able to stand, for a short period, after another attack at 100%.
- Forest Simon Ring: Koyo possesses one of the seven Shimon Rings sealed by the Vongola Sin. When activated, armor covers both his arms and legs, and across his chest. They allow him to use his leaf-shaped Flames of the Forest at full strength. With them, he can manipulate plants such as leaves and vines to cut and restrain opponents.
- Forest Flame: Koyo's Simon Ring emits the flames of the forest, which take on the appearance of leaves. These flames have the power to control and grow vegetation. Koyo can efficiently control the flames at will creating entire arenas of barbed vines or attacking with a blazing storm of leaves that cut rather than burn. When fighting, his main strategy is to use the leaves as a torrential attack in which a tornado of razor sharp leaves surrounds an opponent. Any misstep and the opponent will end up being shredded by the leaves. This torrent also 'blinds' the opponent and allows Koyo to attack from any direction.
- Killer Point Vision: Koyo has incredible eyesight that gives him a heightened sense of his surroundings. Not only this but he can also see his opponents Killer Points and hone in on them using his exceptional boxing reflexes. However the information processed can cause his eyes to overheat if used for a prolonged period of time. As such when not fighting seriously Koyo wears glasses that limit his view to that of a normal human.
- Koyo can be translated as Red Leaves, while Aoba means Green Leaves. This is a reference to his Forest Flame and his battlefield being in a forest (where he claimed the vegetation [green] "was regrown after absorbing the blood [red] of the dead First Simon").
- In the Reborn! Fandom, he is known as 54.
- He slightly resembles Nobuchika Ginoza, a character from Akira Amano's other work, Psycho-Pass.