He first appears when he joins the baseball club, where he wants to play as pitcher. Later, he was seen practicing baseball together with Takeshi Yamamoto (the rest of the teams ran off when they knew Kaoru was going to be coming). He commented that he was nervous when other people watched him play. But, he was able to play after Reborn and the others wore vegetable costumes made by Haru Miura and cheered for him. When he threw the ball, the ball vanished as Yamamoto caught it. This made Reborn wonder what he was capable of and only furthered his theory that the Simon Famiglia wasn't as weak as they were rumored to be.
While he and Yamamoto rested in the locker room after practicing baseball, Kaoru's ring suddenly fell and Yamamoto picked it up. Just as Kaoru snatched his ring, a piece of paper fell and it had the Vongola Crest crossed with the Italian word "delitto," which can mean crime, murder, offense, or delict written across it. With Yamamoto suspicious, Kaoru ignited his ring with flames and impaled Yamamoto, leaving Yamamoto In a pool of blood, badly injured. Kaoru leaves the locker room shortly after. Yamamoto managed to write the word "delitto," or "sin," on the wall with his own blood. Reborn had some suspicion against Kaoru because he was with Yamamoto at that time, but he comes up with an alibi that he had left earlier and Yamamoto had been attacked while he was gone. The next day, Kaoru was later seen at the Vongola Inheritance Ceremony with the rest of Simon Famiglia, who were all surprised to see Yamamoto. 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 was broken open, revealing Kaoru and Enma with the rest of the Simon. Enma stated that they came to the ceremony to take back the "Sin" for the Simon Famiglia, and to get revenge on the "Vongola". Tsunayoshi Sawada asked Enma why they attacked Yamamoto. Enma responded that he was a Vongola Guardian and it was a "rightful atonement." Enma then went on to explain that the Sin belonged to the Simon Famiglia and that it was the blood of their First Boss. Enma declared that he would take over their Famiglia as the Simon Famiglia and take revenge on the Vongola by gathering Mafia from all over the world and standing above them as their leader. The purpose of the war was to gain back the pride of the Simon. Kaoru and the others then released their rings as Enma said that the reason that Vongola Primo got rid of the Simon was because they had a power that rivalled the Seven Flames of the Sky, which were the Seven Flames of the Earth.
Enma then went on to defeat Tsuna and his Guardians. The Simon Famiglia then took their leave and take Chrome Dokuro with them back to their home. After when Tsuna, and the others have recovered, they head of to Simon Base to get Chrome back, and to stop them. Enma and the other Simon members greet Tsuna and the other Guardians. The two families are about engage in battle, but Reborn asks why did they bring those sinster bunch, the Vindice. This surprises the Simon, Adelheid Suzuki asks why are the Vindice here, to which the Vindice reply that they were there to honor the agreement the Simon Primo and the Vongola Primo. The Vindice state that since the two boss treated each other as friends, the two made an agreement to never attack. The Vedice Guardians claim that the two Families would have to fight one another, and the loser would go to prison forever.
The Vindice then take their leave, but before going, Adelheid asks how would they know who wins the battle. The Vindice reply back that if the loser pride is broken, then the other Famiglia wins. The Simon Famiglia tells Tsuna and the others that since this was their home land, they wouldn't engage in battle yet. They would have to follow one road that leads to them. The Simon then leave.
Kaoru, along with his Famiglia and the Vongola, receive memories from the first and second key. Upon receiving memories from the second key, Julie tells him that he need not fight anymore since he fought his counterpart already, Yamamoto.
He along with the rest of the Simon receive memories from the third key.
After Adelheid is defeated by Hibari, the Vindice reveal the 4th key which proves that Giotto attempted to save Cozzato from death but was stopped by Demon Spade. When Hibari notices someone within the bushes, Julie comes out with Chrome and reveals himself as Demon. After revealing his true plans to the Vongola Famiglia and Adelheid, Kaoru appears from behind Demon and stabs him through the stomach, stating that he won't forgive him for what he has done.
But, it was just an illusion and Demon stabs him from behind. He tries to attack him again when Tsuna and his Guardians were trapped by Chrome, but it does nothing and Demon tries to finish him off, but Yamamoto arrives and blocks the attack.
At the beginning of the battle, Demon launced a surprise attack behind Yamamoto. The attack was blocked by Kaoru who said that he knew Yamamoto could dodge it with his own, but said he wanted to pay his debt. Yamamoto called Kaoru an "idiot" for doing that, but he said it was in his own will to help his Famiglia and friends.
It was at this time that the Vindice show up and said that he had lost. Kaoru had given up his pride to protect his Famiglia in order to save Yamamoto, an enemy of the Famiglia. After that Kaoru was taken by the Vindice and the Fifth "Key" was later revealed. The Fifth "Key" was later to be revealed as a set of documents. After they saw the truth of the Fifth Key, Kaoru was taken by the Vindice along with Adelheid. Before being taken away, Kaoru asks Yamamoto if it was okay to be his friend again, and Yamamoto accepts the offer.
At the end of Demon and Tsuna's battle, he and the guardians of both Famiglias are released from the Vindice Prison. He is later seen playing baseball with Yamamoto, accidentally hitting the ball at Enma and Tsuna.
When Adelheid stumbles upon Enma and Skull after them 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, Kaoru 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.