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. Kouyou even encourages Adelheid Suzuki. 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 Kouyou 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. Kouyou is riled up that Reborn speaks the truth so plainly and asks if Reborn has never heard of sugar-coating his words.
Later, Kouyou 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. Kouyou 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, Kouyou starts another argument, complaining about how he will only chase women and gamble instead. Kouyou 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 Kouyou's turn. Due to their competitive natures, Kouyou 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 Kouyou holds the plate while Ryohei breaks it by using Maximum Cannon. Reborn notes that Kouyou is able to endure the punch without suffering any injuries and concludes that the Simon Family is not weak at all.
Kouyou 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. Kouyou 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 Simon 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.
Kouyou 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 Kouyou 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, Simon Cozzato 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 Simon. 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!" Kouyou 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 Dokuro 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 Vindice 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 Vindice 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 Kouyou reveals himself and tells them that in that very area, the blood of Simon Cozzato 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 Kouyou 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 Kouyou 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. Kouyou continues his barrage as Ryohei dodges only enough to get non-fatal injuries. Kouyou 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 Kouyou blind, because his eyes had the ability to see through things and his glasses were made thick enough to control his power. Kouyou then tells Ryohei that before Ryohei dies, he should regret making Kouyou have to use his full power. His eyes then start to light up.
Kouyou 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 Kouyou has Killer Point Vision. Kouyou hits one of Ryohei's Killer Points which makes Ryohei start screaming out in pain. Kouyou reveals that his glasses are a sort of self handicap that keeps his Killer Point Vision under control. Kouyou 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 Kouyou has produced. After telling Kouyou that he won't give up Ryohei breaks free of the vines and tries to rush Kouyou. Kouyou tries to attack Ryohei but his eyesight starts to disintergrate. Ryohei then hits Kouyou point blank with a 60% power Sunshine Uppercut.
Kouyou landed against the barbed ring and it appeared as if Ryohei won. Reborn stated that although Kouyou 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, Kouyou 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 Kouyou can defeat Ryohei before his eyes overheat, or will Ryohei defeat Kouyou with his full power attack. Quickly they charge at each other and it seems if Kouyou is overpowering Ryohei. After all of Kouyou's attacks, Ryohei barely seems to be able to stand properly, making sure he doesn't fall on his knees. Ryohei mocks Kouyou and asks if Kouyou is scared and wondering if he is done with his weak punches. Kouyou, enraged charges at Ryohei and lands a right punch on Ryohei. However, Ryohei takes the opportunity and grabs Kouyou saying he's gotten Kouyou trapped. Finally, Ryohei has his Vongola Gear of the Sun at full charge. Soon after Kouyou 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 Kouyou 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 Kouyou through a portal.
At the end of Demon and Tsuna's battle, he and the guardians of both Famiglias are released from the Vindice Prison.
Kouyou 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, Kouyou 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.