After his first day at Namimori Middle School, Tsuna witnessed Enma getting beaten up after school. Reborn takes full advantage of the situation by pushing Tsuna into the fray, which invariably ends in a sound beating for both boys. Afterwards, when Tsuna helps Enma gather his things, he hands Enma a textbook, which causes both Tsuna's Vongola Ring and a ring that Enma had tied around his neck to start glowing. Enma reacts with surprise and runs off.
Later, Tsuna (under Reborn's instruction) finds Enma at the riverside sewing his pants. Tsuna is mildly surprised at Enma's skill with a needle until realizing that the pant legs had been accidentally sewn together, a mistake highly reminiscent of Tsuna's own Home Economics mishaps. Tsuna returns Enma's textbook, which was dropped when Enma ran off, and both trip and fall into the river. Tsuna invites Enma over to get clean clothes and have Nana sew his pants properly. Enma warns Tsuna to keep away from him if he wants to avoid the perpetual bullying. Tsuna is reminded of his own old, "helpless" demeanor," so he tries to explain to Enma that he is hopeless as well, so he is used to bullying. After an outburst from Lambo, Enma calls Tsuna's home "very lively." Tsuna immediately disagrees, saying that his home is much more "noisy" than "lively." Lambo then starts to hurl grenades at Tsuna, yelling that that their game of "Assassins" will start. Tsuna panics and tries to get rid of a grenade, but is unsuccessful. After the explosion in Tsuna's room, Enma tries to find Tsuna, who is beneath his closet where he hides all his failing exams, making them laughed together.
Enma walks to school with Tsuna on the second day. After Tsuna suddenly recalls the strange sensation he'd had when their rings almost touched day before, he asks to borrow his hand for a minute to see if it would happen again. When they touch, he feels nothing and thinks that maybe he was just imagining things. Gokudera's surprise interruption scares Enma away, running off at high speed. He reappears a little while later at the fight between Kyoya Hibari and Adelheid Suzuki.
It is later revealed that he is a member of the Simon Famiglia, a Famiglia which had ties with the Vongola even before the time of Giotto. He was invited, along with his Guardians, to attend Tsuna's Inheritance Ceremony.
When Tsuna wonders what he should do about the Inheritance Ceremony, Enma suggests running away, which he often considers doing. This shows he has the same feelings about the Mafia as Tsuna does. When Tsuna takes Enma's suggestion to heart, Reborn appears and hits them both, adding to Enma's ever-growing list of injuries. He and Tsuna are then attacked by an assassin sent by the Pesca Famiglia. Tsuna saves Enma by defeating the assassin easily. Reborn, however, senses something unidentifiable from Enma during the first few seconds of the attack, and immediately becomes suspicious. Later, it is revealed that Enma is, in fact, the Boss of his Famiglia. He is shown to have a pendant-like object around his neck and his eyes light up with a Dying Will Flame while looking at it.
When Takeshi Yamamoto and Kaoru Mizuno practiced together, Reborn brought him along, after he saved him from a dog who tried to steal his takoyaki while he was eating it. He, along with the others, cheered Kaoru on by wearing vegetable costumes.
After Tsuna met the Ninth, he stayed at his house because he was on guard duty. Tsuna then consulted with him about his conversation with the Ninth. He asked why Tsuna was telling him this, and Tsuna replied because he didn't have anyone that he could talk to about the Mafia. When Tsuna said that they are alike, Enma said not to put him in the same category because his Famiglia has dealt with a lot of abuse from other Mafia Famiglias because they were weak and small. Enma also said there are times when he feels he could be friends with him because Tsuna is different from other scary Mafia. Hearing this, Tsuna said that he already considered Enma as a friend even without a ceremony; he really thought it was great that they met each other. Before Enma could say something, Nana called them to take a bath. Enma then immediately called Adelheid, and said he really thinks Tsuna is not a bad guy. Adelheid then told him to test him by himself but reminded him not to forget the "Sin of Vongola." Enma then wrote a letter for Tsuna that he was threatened by a Mafia group who took down the Giegue Famiglia and is trying to interfere with the Vongola Inheritance Ceremony. He planned for Tsuna to go to the factory side at noon and wanted his help. He then awoke at night and left the letter on Tsuna's desk and went to the factory. The next morning, he is seen waiting at the factory alone.
Adelheid later finds Enma sitting alone in the factory and says that Tsuna went to school and ignored the letter he wrote. Adelheid tells Enma that the Vongola Inheritance Ceremony will be a success and that it is the only way to restore the Simon Famiglia's power. Enma then responds that he knows.
Attack on Inheritance Ceremony
Enma is later seen at the Vonogla Inheritance Ceremony getting beaten up by one of the guests, who wondered why a no-name Famiglia like the Simon had come to the ceremony, but Adelheid intervenes and says that they had received invitations. Tsuna, who'd seen this, asks if he is okay and wonders where he was yesterday. Tsuna tells him that he'd decided to "go with the ceremony," Enma however appears to give him a disappointing glare. He is later seen in the crowds of the ceremony, waiting along with the rest of Simon Famiglia, where Tsuna will 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, and the open safe revealed Enma and his Guardians. Enma says he'd needed this to take revenge on the Vongola. He breaks open the bottle of Sin and pours the contents (supposedly blood) onto his ring and goes into Hyper Dying Will Mode and his arm mutates into a robotic arm. Tsuna asks why he did this somberly, then he as well breaks into Dying Will Mode and Enma prepares to face off against him. Tsuna asks Enma why they attacked Yamamoto, to which Enma responds 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 then asks Tsuna if he's ever heard the story, which Tsuna replies, "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!" This angers the Simon and they make the declaration that Enma will inherit their Famiglia as the tenth boss and take revenge on the Vongola by gathering Mafia from all over the world and standing above them as leader. The purpose of the war being was to gain back the pride of Simon.
Adelheid and the others then release their rings as Enma says that the reason that Vongola Primo got rid of Simon was because they had a power that rivaled the Dying Will Flames of the Sky, the Dying Will Flames Of The Earth. Enma told Adelheid and the others to stay out of the fight against the five Vongola guardians; with his new found powers, Enma lifts Hayato Gokudera and Ryohei Sasagawa aside, sending them in into the wall. Chrome and Hibari are also lifted in the air, due to the sheer power of the Simon Rings that have been strengthened by the Sin. Just then, Tsuna asks Enma to stop and rushes to stop him. Tsuna asks why he is doing this, to which Enma responds saying that this is his fault, and that he has been watching Tsuna for some time and that he is different from the other previous bosses. Tsuna is than overpowered by him and Enma proclaims that in seven days time, his power will grow, and that he'd only been using 1/7 of his full power. He then left with the rest of Simon Famiglia stating they wanted them to experience hell and that the Simon could kill them anytime.
After Tsuna and the others have recovered, they head off to Simon Base to get Chrome back, and to stop them. Enma compliments Tsuna and his Guardians for finding their base so quickly, and that they all came alone. Reborn asks why they brought the Vindice. The Vindice guards claim that their only reason for coming is to fulfill the promise between the Simon Primo and Vongola Primo. The Vindice state that since they considered one another as friends, the Simon and the Vongola must never fight one another. The two bosses made an agreement to never attack one another, and thus, the Vindice made a law, in which the losing Famiglia would be erased from existence forever, and spend the rest of eternity in Vindice Prison. The Vindice state that since this is the second time this happened, they must fight one another. Enma receives memories from the first key; he calls for a meeting within the Simon Famiglia.
Upon receiving memories from the second key and learning that Large was defeated, Enma becomes very depressed and upset, demotivated in continuing the fights. However, Julie manages to manipulate Enma into continuing.
After Shitt P! is defeated by Gokudera, Enma attempts to save her by using his weapon,but he was too late. Agitated and angered further, he challenges Tsuna then and there to end it. As Enma laments how Koyo, Large and now Shitt P! have been imprisoned the Vongola have again ruined his Famiglia, Tsuna remembers Ryohei, Yamamoto and Chrome and claims that Enma and the Simon are doing the same thing to him, and enters Hyper Dying Will Mode. The two then engage in a fierce battle, seemingly evenly matched, but when Enma proclaims that it was Tsuna's father who killed his parents and sister, Tsuna is frozen. Enma then grabs Tsuna by the throat, preparing to finish him off.
Just as he almost reaches Tsuna's neck, his weapon backfires, causing him to be injured. He falls and passes out as Adelheid comes in time to support him from the backlash.
Later it is revealed to the members of the Simon and Vongola that it was due to Demon Spade's deception that the first generation Simon boss was killed. Enma did not react well, despite a new memory showing that Cozzato was saved by Primo's Guardian's. Adelheid mentioned only Tsuna can save "Him," now titled as the "King of Simon" and his only goal is to kill Tsunayoshi Sawada, at all costs. During their battle, Tsuna can hear the voice in Enma whispering for Tsuna to "Save" him and that he doesn't "want to be alone." After unleashing his Gravity Spheres unto Tsuna and creating a field of black holes, causing Tsuna having to use his Cambio Forma Ver. X, Enma realizes that Tsuna has come to rescue him, just like the Vongola Primo came to rescue Simon Cozzato.
He then snaps out of it, and thanks Tsuna for saving him. After learning from Tsuna that his pride is his friends, Enma included, the Vindice appear declaring Enma the loser, but refrain from imprisoning him in Vindice Prison as the battle of the Guardians is not finished, and as a boss Enma must witness it to the end. Enma then witnesses Mukuro's battle with Demon Spade. After Demon is seemingly defeated and Julie is freed, he rushes to Julie's side as he awakens and Mukuro, now possessing the Mist Owl, declares that he cannot return to his body and that Demon Spade must have taken it while he was possessing Chrome.
The Vindice then appear alerting them that Demon Spade has indeed taken control of Mukuro's body and has broken out of the Vindice Prison and shall make his way to kill the Vongola. When they are asked to take care of Demon Spade Tsuna agrees to do so only if all of their friends are released since there is no more misunderstandings and ill-will between the two Mafia Famiglias. Enma supports this and agrees to this as well and the Vindice agree as well, willing to do so once Demon Spade is defeated.
Demon arrives and quickly displays his power, revealing he has all six of the Simon rings and the Vongola bangle of the sun and is capable of using the other Vongola Gears as well. All of Tsuna's guardians are quickly defeated and trapped within illusionary realms. Tsuna steps up to fight and Enma joins him. Demon tries to trick Enma once more by talking about the death of his family at the hands of Tsuna's father. However Enma tells him to shut up because he will no longer believe his lies and that he will never betray Tsuna again because he knows Tsuna will never betray him. Demon then reveals the truth of the incident, that it was all set up by him and that he killed Enma's family while posing as Tsuna's father. Enma and Tsuna are both enraged and complete a combo attack which hits Demon, despite not planning anything ahead of time but just in that moment.
As the battle finally begins Reborn states that Enma and Tsuna are no longer the completely No Good Duo instead they are the Hyper Duo. Realizing the combined power of Tsuna and Enma, Demon decides to get serious and uses all six Simon Rings and Vongola Gears and multiplies himself into six using the cloud's attribute of propgation, separating Tsuna and Enma to prevent them fighting together and coordinating their attacks, enabling Demon to overwhelm both of them. When Dameon reveals that they need to attack all six of him at the same time, Enma decided to use his remaining Flames to create six blackholes and catches all six Demon, gathering him around him and tells Tsuna to incinerate Demon into ashes along with him as he says that this is the only way to defeat Demon.
Fortunately for Enma, Chrome jumped to his side and protect him with full power Cortina Nebbia that also supported by Mukuro's flames to strengthening the barrier. Convinced that he would be safe, Tsuna fires the XX Burner. As the smoke from XX Burner cleared up, Demon, who turned out to be alive, disguised as Enma and begun to attack Tsuna by breaking his bones before Enma could warn him in time. As he watched Tsuna about to be killed, Enma desperately tried to move to stop Demon. As he stated that Tsuna is his first friend outside his Famiglia, Enma's Earth Simon Ring came off and flew to Tsuna's Vongola Ring, combining and form into new Ring, shocking everyone including himself. Using the newfound power of the combined Sky and Earth Flames, Tsuna defeats Demon Spade, securing Enma's freedom from Vindice.
Enma listens to Demon Spade's story, and receives the final 'key' from Vindice with the others. Having finally made peace with the past and Tsuna, they move forward. Later on, he is seen running from a dog on the way to school, coincidentally dragging Tsuna into his predicament. The two then get hit by Shitt P!, who chases after Gokudera, and later on a baseball consecutively hits them both on the head. Adelheid punishes the two for being late (pushed by her anger with Julie's flirtuous attitude) by hanging them upsidedown in nets from the school roof. Reborn notes that their idiotic actions would've been liked by Cozzato and Giotto, and the two laugh together.
Enma is shown to be heading to the river in Namimori to feed the cats that normally ask him for food, but find that Skull had already fed them a large amount of cat food. Enma recognizes Skull as an Arcobaleno when he sees Skull's Pacifier after Skull demanded that Enma fight for him in exchange for feeding the cats. Later, Enma is seen at home begging Adelheid to help Skull, although not understanding what Skull was asking them for help for, and later being ordered by Adelheid, who misunderstood just like Enma, to tuck Skull into bed.
Later, he meets Tsuna and Reborn, who were about to ask him to join their team, but find out that Enma decided to be Skull's only representative. He asked Tsuna for alliance, but both Skull and Reborn immediately refused, much to Tsuna's dismay. During the first day of the Representative Battle, Enma was attacked by Varia and was being overwhelmed. When the Varia was about to deliver a finishing blow towards the helpless Enma, he was saved by Skull, whose Arcobaleno Curse has been temporarily removed, using his Undead Body ability. Afterwards, out of gratitude, albeit reluctantly, Adelheid agrees for the rest of the Simon Ring Holders to become Skull's representatives.
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. At the end of the third battle day Tsuna arrives in Enma's hospital room asking for help. The next day, Enma, along with other people whom Tsuna had contacted gathered at Tsuna's house to discuss plan to defeat Team Bermuda.
On the fourth day, Enma, Tsuna, and Basil are chosen as Commando until responsible for defeating the Vindice one by one, starting with Alejandro while the other elite members hold up Jaeger, Small Gia, and Big Pino. With their teamwork, the trio able to defeat Ginger Bread and destroys Alejandro wrist watch. They immediately went to where Gokudera, Yamamoto, Chrome, and Fran are. Enma used his gravity manipulation to save the four of them from Big Pino's attack. He binds Big Pino and Small Gia while Yamamoto and Gokudera restrains them close enough to destroy the watches. With Chrome and Fran's illusions, Yamamoto and Gokudera are covered by layer of ceramic right before the dynamites explode, destroying the Vindice's watches without harming Gokudera and Yamamoto.
After resting for three minutes from their battle against Small gia and Big Pino, they immediately fight Jaeger who had already beaten most of the elite team members, leaving only Mukuro and Hibari. Enma stopped Jaeger's attack with his gravity manipulation when the latter tried to attack the former. However, he was quickly defeated by Jaeger along with Chrome and Fran. Before he went unconscious, he lended his Earth Simon Ring to Tsuna. After Bermuda and Jaeger were defeated, he was taken to Namimori Hospital with the other injured members. In the final chapter, he is shown with his Family when Tsuna thought that now he has friends that he could rely on when necessary.