Ryohei is always hiding the truth of his actions -the actions relating to the Mafia- from Kyoko, due to an incident that had happened while they were in primary school. Some high school students who did not like Ryohei used Kyoko to lure him into a trap. In his attempt to rescue her, he suffered severe injuries, which resulted in the scar on his left temple. Kyoko still feels responsible for this incident, though she accepts that he needs to fight. He is deeply concerned for Kyoko's safety, so he covers up his actual life-or-death battles with the cover story of "Sumo matches". He is known to have great boxing skills in the Vongola Famiglia.
Ryohei was first introduced when Tsuna accidently pulled him to school while in Dying Will Mode. Ryohei was instantly inspired by Tsuna's spirit and urged him to join the boxing club. They later fought at the boxing club where they were both shot with the Dying Will Bullet. However it had no effect on Ryohei as he always lives his life to the limit. All his future appearances in this arc was to make Tsuna join the boxing club.
Ryohei was targeted by the Kokuyo gang due to his status as Namimori Middle School's fifth strongest. He was badly injured by Ken Joshima and lost five teeth, resulting in him being hospitalized. He later lied to his sister Kyoko as to how he received his wounds. Later, during the fight with Mukuro Rokudo, Ryohei was one of the people Tsuna saw while under the effect of the Criticism Shot.
A month later Ryohei was chosen as the Guardian of the Sun and was given a half Vongola Ring. In order to prepare for the Varia's oncoming arrival, Colonnello became Ryohei's home tutor, and the two departed to a mountainous region. It is during this time that Ryohei trained by relaxing his body as opposed to exerting himself. Colonnello also shot Ryohei with a special bullet that risked death but ultimately allowed him to unleash a mighty blow called Maximum Cannon. Later in the week Ryohei, Takeshi Yamamoto and Hayato Gokudera, defended Lambo from Levi's Lightning Squad.
Before the battle Tsuna worried about Kyoko's reaction to her brother fighting, however after telling him about how he got his scar, Ryohei made him promise to keep it a secret from her. Later Ryohei became the first to fight, in the Sun Ring Battle against the Varia martial arts expert Lussuria. Ryohei who was immediately disadvantaged due to overhead floodlights in the Sun Arena, preventing him from seeing (Lussuria already had sunglasses), then had both fists broken after punching Lussuria's Metal Knee. However, once Kyoko arrived, Ryohei gained enough motivation to smash Lussuria's metal knee as well as his leg, ending his use of Muay Thai. Gola Moska (after assuming his loss) then shot Lussuria leaving Ryohei the victor. During the Sky Ring Battle after being cured by Gokudera, Ryohei gave Lussuria the antidote despite being his opponent. He later saved Yamamoto, Chrome Dokuro and Gokudera from Mammon's illusion by destroying the entire gym with his Maximum Cannon. Later the Vongola Guardians forced the remaining Varia to surrender.
In the Future, Ryohei had gone to work with the Varia in Italy. When he returned, he found Chrome after her fight with Glo Xinia at Kokuyo Healthy Land and brought her to the Vongola Hideout. He also brought along information concerning the plan to attack the Millefiore Family's Base (as well as presents for the children).
He later joined the others during the Merone Base invasion. The group were eventually discovered by Vice Captain Ginger Bread who claimed to be responsible for Colonnello's death. Though greatly angered by this, Ryohei allowed Lal Mirch to fight. However he was revealed to be a puppet and the base was alerted.
The group later found the Black Spell member Nigella Beabankul as he was finished off by White Spell Baishana. Ryohei (angered at Baishana's disloyalty) crushed him with his own Box Weapon. Soon after the Merone Base was activated, Ryohei and Gokudera were separated from the others and encountered Gamma. Ryohei, who had tied Gokudera up in order to prevent him rushing into the fight, was knocked unconscious during his battle with Gamma. However, thanks to Kangaryu's activation ability, Gokudera's Box Weapon grew up, giving him the upper hand in the coming fight. Later, Ryohei was switched to his Past self in order to unite the Vongola Rings, creating a momentary force field around those present. Then (along with the other Guardians) Ryohei received a Vongola Box Weapon from his future self and had the situation explained to him.
When returning to the past for the Arcobaleno Trials, Ryohei was chosen, along with Tsuna and Yamamoto, to take part in Mammon's Test of Adaptability. Under the belief that Kyoko was being held hostage, the trio were forced to solve a series of riddles as well as fight a number of Box Weapons. On the way Ryohei blindfolded himself, as he could find his way through the illusions based on his training route. After discovering that Kyoko was safe, they swiftly defeated Mammon and passed his trial. Later Ryohei 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.
Upon returning to the future, Ryohei was taught by Gokudera how to use the Rings and Box Weapons. He was adamant about keeping Kyoko in the dark about the Mafia, but upon learning that her attendance to Choice was mandatory, he forgave Tsuna after a single punch. After the Vongola lost the Choice battle, Ryohei healed Shoichi Irie who had been fatally wounded in the fight.
After being sent back to the past for the Inheritance Succession, Reborn became Ryohei's tutor and they began training in order to release the pent up energy brought about by Ryohei's impatience. During Kyoya Hibari's trial, Ryohei was adamant about convincing him to participate and followed him to the school roof. However, they were interrupted by Hibari's trainer, Skull, in his airship who, frustrated by Hibari's lack of interest, began to attack. During the battle Ryohei helped Hibari protect the school resulting in Hibari taking his trial in order to repay Ryohei, however First Sun Guardian (Knuckle) appeared from the Sun ring and announced that Ryohei had already passed his test due to standing against adversity without fear and convincing Hibari to participate. The First Cloud Guardian (Alaudi) also appeared, stating that Hibari had also fulfilled his duty. Later, due to their involvement with the Mafia, Kyoko and the girls were trapped by the First Generation Mist Guardian (Demon Spade) at Healthy Land. Ryohei and Lambo were separated from the others while attempting a rescue and were forced to fight doppelgangers of themselves. The illusion was then torn apart by Hibari's rampage.
Ryohei was later hurt while trying to save Uni from the Six Funeral Wreaths. Despite this he flew up with Reborn in order to backup Tsuna leading to the Funeral Wreath's retreat. That night Ryohei agreed to protect Lambo in the coming battle. The next day the two of them along with Basil, Tazaru, and Nosaru encounted Kikyo at the lake. Ryohei then undid the light hypnosis on Lambo causing him to destroy Kikyo's army of Velociraptor di Nuvola with his Vongola Box. After Kikyo opened his Asura Box, Ryohei asked the others to sit back while he fought. By activating his Cambio Forma, Ryohei was able to catch Kikyo of guard but due to his previous injuries, he was unable to defeat him. He then appeared to be defeated by Kikyo but was revealed to be an illusion created by Mukuro. Later during an all out battle between the Vongola and the Six Funeral Wreaths, Ghost suddenly appeared and Ryohei tried to combine an attack with Gokudera and Basil. However the attack passed straight through him and Ryohei (along with everyone else present) had his Dying Will Flames drained. The Vongola eventually defeated Byakuran and returned to the past.
After returning to the past, Ryohei soon got into a fight with Kouyou Aoba, one of the transfer students from Simon Middle School. After the Simon revealed themselves to be an allying mafia family Ryohei was grouped with Kouyou in order to protect Tsuna. When no assassins arrived, Reborn made them compete with a set of puzzles on a plate made of extra hard ceramic which were too difficult for either of them. Reborn stated that the competition would have to be postponed if the puzzles were to break. Therefore, Kouyou held the plate, while Ryohei broke it by using Maximum Cannon, which Koya was capable of receiving. Later Ryohei was the first to find Yamamoto after the attack and called the ambulance and started to heal him. However he noted that the wounds were too deep. During the Inheritance Ceremony, the Simon betrayed the Vongola and Ryohei (along with the other Guardian's) was defeated by Enma Kozato and his Ring was broken. However Talbot then appeared and repaired the Vongola Rings by combining them with the Animal Rings. With Ryohei's resolve, his ring turned into Vongola Gear.
Ryohei later joined Tsuna, Gokudera and Lambo as they invaded the Holy Land of the Simon. While traveling they were attacked by Kouyou to whom Ryohei demanded a one on one boxing match, in a ring of Kouyou's barbed Forest Flames. However, so much as placing a knee on the ground would result in a loss of their pride as boxers. Kouyou then used his Flame to restrict Ryohei's movements and beat him from all angles but by using his Vongola Gear Ryohei was able to turn the damage he had received into his own power. However in the process Kouyou's blacked out glasses were removed allowing him to use his eye sight to its full potential, though Ryohei was able to figure out Kouyou's weakness and strike a critical blow. Despite his injuries Kouyou was still standing and the two squared off in a match of endurance. After having each of his Killer Points destroyed by Kouyou, Ryohei delivered a devastating attack at full power that cleared the forest. However the two remained standing, each refusing to admit defeat and after crossing fist's once more, both collapsed at the same time. The Vindice then arrive and imprisoned Ryohei and Kouyou as they had both lost their pride. With his last words before vanishing into the portal, Ryohei encouraged Tsuna to find his own pride and to stick by it no matter what. After Demon Spade's defeat, he is set free, along with the Simon. Later, he is seen at school caught in Adelheid's nets with Kouyou and Julie, for breaking school property in a spar with the former.
Ryohei appears with Tsuna at Chrome's welcoming party after Chrome was kicked out of Kokuyo Land and transferred to Namimori Middle. In the mimicking competition, he mimicked I-Pin and Reborn, but was kicked off by Reborn, who told him to "get lost to the extreme!" while doing so.
Later, on the first day of the Representative Battle of the Rainbow, Ryohei happily heads to school with Tsuna. However, when the battle suddenly starts, he, Yamamoto, and Gokudera 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 Hibari and Fon, the former appearing to be Fon's representative, who states that he would bite all three of them to death. Ryohei activated his Vongola Gear, but his watch was easily broken by Hibari before they could even start the fight, eliminating him from the battle. They later discussed the result of the battle and their next plan for the next day battle.
A day before the final battle against Team Bermuda, he, along with the other people Tsuna has summoned, gathered in Tsuna's house to arrange plan to defeat Team Bermuda together. He and Fon were pointed as medical squad for the Allied Team, healing Basil's injuries after defeated by Small Gia and Big Pino, and later watched the rest of the team members fighting Bermuda and Jaeger. After Tsuna defeated Bermuda, Ryohei and the others provided enough flames for the Pacifier Jars created by Talbot, so there won't be anymore Arcobaleno. Not long afterwards, he visited the hospital along with Tsuna, Gokudera, Yamamoto, Reborn, and the girls to check up on Dino and Basil.
In the final chapter, he is last seen with Gokudera, Yamamoto, Chrome, Lambo, Kyoko, Haru and I-Pin in front of Tsuna's house, waiting for Tsuna as they are invited by Reborn to celebrate Tsuna's tutelage to become Neo Vongola Primo.