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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 Suzuki intervening. Still, Koyo seems to have bitter relationships with his Famiglia members. One such example is Julie Kato, 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.
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 Simon 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.