As revealed in a flashback sequence, Mukuro Rokudo was born into the Estraneo Famiglia and was one of the children experimented by the Estraneos. Five years prior to the main storyline, Mukuro killed his captors with his abilities and offered Chikusa Kakimoto and Ken Joshima, two children who were also experimented on, a chance to join him in his new quest to destroy the "insignificant" world.
Later on, Mukuro was adopted into a Mafia Famiglia in Northern Italy. However, unbeknownst to the Famiglia, Mukuro was periodically controlling one of its Members, Lancia, and using him to kill others, eventually making him kill his own Famiglia and several Mafia Famiglias in Northern Italy. Since then, Mukuro had Lancia under his complete control, using him as a "Fake Mukuro" who is eventually mistakenly identified as the real Mukuro in a picture, whereas the real Mukuro is said to have never been photographed, although on two occasions, the real Mukuro has been shown to have been photographed.
Anything else Mukuro did in recent years remains a mystery, but it is mentioned that he, along with Ken and Chikusa had been locked up in a high security Italian prison, which is reserved for the most dangerous of Mafia criminals who have even committed crimes against the Mafia itself. Two weeks prior to the Kokuyo Gang's appearance in the story, they staged a jailbreak on the eve of Mukuro's execution, resulting in the death of several wardens and prisoners, and later the escape of other dangerous criminals. Afterwards, they attended Kokuyo Middle School as foreign transfer students, and within a week were able to take over control of the whole school.
Mukuro is first seen during the Daily Life Arc when Chikusa reports that he found Ranking Futa.
After creating the Kokuyo Gang, Mukuro kidnapped Futa and used his rankings to draw out Vongola 10th; he also defeated Kyoya Hibari by taking advantage of his weakness to cherry blossoms. Later, he revealed his plan to take over Tsuna's body in order to get revenge on the Mafia. After being defeated, he and his gang were arrested by the Vindice. One month after the battle when Tsuna and the others cheered Yamamoto at his baseball game, Tsuna sensed something but was distracted by Lambo who started crying. Reborn immediately noticed a little boy, whose right eye was the same as Mukuro's eye, indicating that he was possessed. He stated quietly that Tsuna and himself would meet again.
A month before Varia's arrival, Mukuro, Ken Joshima, and Chikusa Kakimoto attempted a jail break. Mukuro used himself as bait so the others could get away and as a result, was thrown into the lowest level jail cell. He later discovered Chrome Dokuro, and became the Vongola Mist Guardian in exchange for his comrades' safety. During the Mist Ring Battle, Chrome was defeated by Mammon. Mukuro then appeared briefly and defeated Mammon himself. It is revealed by Chrome that Mukuro knew Xanxus's plans and asked Lancia to help them after the battle.
In the future, Glo Xinia claimed to have defeated Mukuro during a previous battle. However, Mukuro had in fact possessed Glo's Owl Box Weapon and used this to help Chrome defeat Glo Xinia at Kokuyo Land. He spied on the Millefiore under the disguise of Leonardo Lippi, sending information of the Millefiore to the Vongola, but ends up being injured in a fight against Byakuran, who claimed to have always known his true identity. Mukuro planned to leave his body after gathering information about Byakuran's power, but was stopped by him as he blocked out Mukuro's power to transfer his consciousness out of Leonardo's body. After this, Mukuro's fate is unknown; however, he managed to leave a message about Shoichi Irie's round device to Chrome and Tsuna in their dreams.
When Tsuna and the others came back to the past to complete the Arcobaleno Trials at Kokuyo Land, Mukuro communicated with Chrome and asked where she had been for past few days. Seeing Chrome's reluctance to answer, he let her keep the secret for herself and told her he will always by her side. After Colonnello's Trial finished, Mammon, disguised as Tsuna, appeared in front of Chrome and yelled at her to bring out Mukuro. Mukuro appeared out of anger, asking who was bullying Chrome. Mammon baited Mukuro to come outside, where the real Tsuna came to give Chrome rice balls, and where Mukuro will arrive at the same time as Hibari. Hibari and Mukuro then immediately battled each other rather than attack Tsuna.
After Tsuna defeated Skull's octopus, he was able to stop Hibari and Mukuro's battle. He revealed that he had known all along of Mammon's disguise and merely playing along since he thought it would be amusing. He then requests Tsuna to not drag Chrome around outside his reach, and if that was not possible, to then protect her. After Tsuna promised this, he disappears, because he had already reached his limit.
He later appeared again while communicating with Chrome when she returned to Kokuyo Land after Verde's trial, and asked whether she was involved again with Vongola and Arcobaleno activities. However, he again let her keep it the secret for herself since all that mattered was that she was safe.
After Tsuna decided to protect Uni, the Vongola attempted to get back to Namimori. As Byakuran approached them, Mukuro appeared as a real illusion from Chrome's trident. He uses his illusions to slow Byakuran down. Mukuro's intervention successfully allowed the Vongola to escape. However, to Byakuran's surprise, he states that he helped them only to keep Uni out of Byakuran's hands, hinting that he knew something before his illusion was destroyed.
While Tsuna was trapped inside Demon Spade's illusionary mist, he fired a weak X-Burner at Demon, but it turned out that he was in fact firing it at Chrome, Haru, I-Pin and Kyoko. In an instant, Mukuro appeared in Chrome's stead and protected them, stating that he couldn't let other people run wild in his territory. He turns his attention to Demon, angry at the latter for torturing Chrome few days ago, revealing he noticed this because he had been observing Demon for a while, but did not interfere until Demon had pushed him too far. When Demon asked what he intended to do, Mukuro replied that he could deal with him directly, but would let Tsuna handle it instead. Tsuna attacked Demon, who would have gotten absorbed into the illusionary mist again if Mukuro hadn't gotten rid of it, allowing Tsuna to fire his X-Burner directly at him. After reappearing, Demon intended to question for Mukuro, whom he referred as the Mist Guardian, much to Mukuro's disapproval. He tells Demon his dislike to be referred by the title as he did not earn that title by choice and revealed his goal to destroy the entire mafia and conquer the world, much to Demon Spade's shock and amusement. When questioned why he saved the girls, he simply replied that Chrome would be upset if they got hurt and he personally doesn't care whatever will happens to them. Seeing that Mukuro has more potential than the other Guardians and fulfil the role of Mist Guardian, Demon deemed him worthy of his inheritance. Mukuro, however, who has no interest with it, returned to his own body as he entrusted Chrome to Tsuna.
Mukuro was rescued from the Vindice Prison by Ken, Chikusa, M.M., and Fran. During an all-out battle between the Wreaths, the Varia, the Vongola Guardians, Mukuro and his student, Fran, intervened by swapping the Vongola members with illusions in an attempt to learn more about the Wreaths' fighting styles and abilities.
This plan worked, as they were able to witness some of the Wreaths' special attacks without suffering any casualties. He later received the Mist Vongola Box Weapon from Chrome, which he used to analyze Ghost and was shocked to find that he is not a human. During Tsuna's battle with Byakuran, he and the others are forced to watch as they already used up most of their flames. After Tsuna defeated Byakuran, he was seen with Chikusa and Ken calling out to Chrome since it was almost time to go back to the past and told her that it was good seeing her again, calling her Nagi. Later, his past counterpart was seen receiving the memories from that Future's battles from Uni. When Chrome went back to Kokuyo Land, he switched places with her and commented that this would be interesting, while he looked at his Vongola Ring and Vongola Box Ring.
When Demon undone his mind control on Chrome, he revealed that he wishes to fight Mukuro. He proceed to remove the barrier of the Simon Island and the illusionary organs from Chrome. He shouts for Mukuro to come out, taunting him that Chrome will die unless he saves her. Mukuro immediately appears, replacing Chrome. Furious at Demon, he comments that Demon has lost sight of the times of his death, calling him a putrid rotten mafia.
Demon comments that with one glance, he could tell that Mukuro is an amazing technique user, to which Mukuro replies that he could see the same and calls him the epitome of everything he detests. This amuses Demon who continues to saying that while he understands why Mukuro takes offense, emotions like anger can weaken him and instead offers him to have some tea to calm him down. Mukuro quickly declines this and states that he will take him into the realm of the dead.
After witnessing the sixth key, Mukuro and Demon clash with their weapons as Demon comments that Mukuro is better than he had ever thought, then deciding to not hold back, to which Mukuro replies that he wouldn't have it any other way. When Demon reveals his cards and explains that his illusions are stronger than him due to the combined flames of Mist and Desert, Mukuro responds by revealing the ore of the Mist Vongola Gear and upgrades it to the Mist Earring Version X and commands Mukurou to Cambio Forma, changing his Trident into a khakkhara. Mukuro traps Demon within his illusions and manages to make Demon lose his perception, defeating him. As Demon turns back into Julie and become unconscious, Mukuro bids him farewell. When Demon finally disappears, Mukuro goes back to his original body, telling the arrived Tsuna to take care of Chrome.
However, he later surprises everyone in the room when he possesses Mukurou, the box animal and starts talking. He is shocked himself because while he was on the verge of going back to his body, he was cut off. That leads him to the conclusion that when he was possessing Chrome's body, Demon took advantage of his own soulless body and possessed it for his own before he does.
When Demon finally arrived back with new appearance and power, Mukuro tells Tsuna to destroy Demon no matter what the consequences to his body. Stating that Demon Spade's soul has fused with his body and has become a monster and they will lose everything if they didn't fight him with everything they have. As Tsuna hesitates to shoot Demon in afraid of killing Enma when he catches Demon, Chrome jumped to Enma's side and creates Mist Barrier with Full Power so Tsuna would shoot. Mukuro interjects that her flame is not strong enough to defend XX Burner and supports her with his Flame, making him exhausted. After Demon was defeated, Mukuro returned to his own, beaten-up body.
After the battle, the Vindice freed Mukuro from their prison due of his efforts in helping to defeat Demon. One week afterwards, Hibari is said to have gone to challenge Mukuro at Kokuyo Land, leaving Namimori Junior High temporarily in Adelheid's care. Unfortunately for Hibari, Mukuro was not present at that time.
Search for Fran Edit
Mukuro left Chrome behind in Kokuyo Land with a letter that telling her to get out and a Namimori-Middle uniform. Shortly after, he convened Reborn to talk about Chrome and explained that he wanted to get ahold of Fran who was living in France and thus, leaving Chrome in Namimori along with her living expenses, but not telling his reason for leaving her.
When they arrived in countryside of France called Jura to recruit Fran to their team, they ended up meeting Varia. When they found Fran, Mukuro approached him as his master but quickly furious once Fran mistook him as a fairy and compared his hair with pineapple. After finding out that Fran has lost his memories of his future-self and cannot stand with Fran's constant mockery, both he and Squalo said to each other that they could have Fran. Mukuro refused to take Fran into custody with excuse that he's busy and cannot stand his stupidity that needs to be cured first with Varia's facility and then afterwards, he will get him back. After the arguments with Squalo, they decided to conclude with a game of ladder with the loser must take Fran into their custody. However, Fran interrupted and stated that he already decided which group he will go with, later revealed that he has chosen Kokuyo.
Declaration of War Edit
Afterwards, Mukuro and his gang went back to Kokuyo Land and he was seen talking with Verde, who asked him to listen to his favor that Mukuro found interesting. Mukuro then asked Verde what he should do. The next day, Mukuro and his gang approached Tsuna, declaring a war against them as he had sworn to himself that he would defeat Tsuna the moment he was freed from Vindice Prison. Tsuna, however, refused to fight him. Knowing this, Mukuro replied that he would fight him nevertheless if it was for Reborn's sake. At this, Verde appeared and revealed that he was the one who brought them to Namimori, requesting the Kokuyo Gang's assistance for the Arcobaleno fight. Mukuro expressed his interest in Verde's intelligence despite their mockeries towards each other, which Verde returned the same and stated that he wished to defeat Tsuna, and destroy Vongola Famiglia. Before he left, Tsuna questioned him about Chrome and surprised when Mukuro answered that they can look after her as he detested the current Chrome, telling them to take good care of her and left.
Later, a day before the Representative Battle, Mukuro is seen with Verde when the latter shows him his new inventions for the battle, his alleged "weapons of the next generation".
1st Day of Representative Battle of the Rainbow Edit
The next day, during the Representative Battle, while he and his team are fighting Team Colonnello and Team Uni, he sends his illusion to Chrome. He tells her that he came not to take her back with him, but to check if she was getting better, but found that, as he had expected, her condition has worsened. He demands her to tell him everything about her current condition; otherwise, he reveals that she was going to die, shocking Chrome. He continued saying that if her condition gets even worse, she would only have about three days to live in which her health would deteriorate rapidly as he revealed the cause of her illness.
Afterwards, he was seen back at Kokuyo Land discussing the result of the battle so far and praised Verde's machine. However, Fran then interrupted, saying that both of them said insincere things that they didn't actually mean it and mocked Mukuro's hair as pineapple hair as he made his apple hat into pineapple shaped hat, much to Mukuro's annoyance who then started to stabbing his hat and increased his illusionary training twice from before as punishment. He then said to him that he wants to use their new equipment at the second day of Representative Battle with his help, to which Fran willingly agrees to.
2nd Day of Representative Battle of the Rainbow Edit
At night of the second day of the Representative Battle of the Rainbow, Mukuro, along with Fran, Ken, and Chikusa attacked Reborn's team and Uni's team. He disguised himself as Kikyo, destroying Zakuro's wristwatch, and then encountered Tsuna, fighting him. When Colonnello launched Maximum Rifle, he managed to protect his wristwatch from being destroyed, but was injured in the process. With Fran and himself injured, as well as Byakuran's boss watch destroyed and Tsuna left to confront Iemitsu, Mukuro saw no merit in continuing to participate in the battle and retreated with Fran.
Vindice's Assault Edit
Not long, however, shortly after Checker Face announced Team Bermuda's participance, a Vindice appeared in Kokuyo Land and attacked Mukuro and his gang with intent to kill, leaving him no choice but to fight. Mukuro was saved by Fran's illusion and protected his boss watch, but heavily injured. Verde told Mukuro that he could quit if he wants to as the situation has became more dangerous and he wanted to devote himself for his research, but Mukuro refused, saying that there's still something that he needs from the Representative Battle of the Rainbow. Shortly, he left Kokuyo Land to go to Namimori town, and during his way, his watch suddenly announced one minute to prepare for the next battle.
3rd Day of Representative Battle of the Rainbow Edit
At the park, he encountered Tsuna, Reborn, Yamamoto, and Gokudera. They questioned what he was doing in Namimori, but he didn't feel like answering them and ready to fight. Mukuro's team arrived just in time as the watch announced that the time limit would be twelve minutes. Ken and Chikusa able to rejoin the battle by using the spare watches that hadn't been used. When they were about to start, however, three Vindices arrived. Verde suggested that they should temporarily team up with team Reborn, but Mukuro refused, confidently saying that with Ken, Chikusa, Fran, and Verde's invention, they could win against both team Team Reborn and Vindice. Fran, unfortunately, was asleep. Ken tried to wake him up, but Fran is sleep talking and didn't wake up by Ken's call. The Vindices attacked Mukuro and Tsuna, but both able to block their attacks. Having no other option, Mukuro and Tsuna decided for a temporary truce and fight the Vindice together temporarily. With combined efforts of both teams, they able to rip off Vindice clothes and witness their true forms with stone pacifiers similar to the Arcobaleno's each around their necks. The Vindice, Jack, complimented them as the first people to be able to rip off their clothes and saw their true forms before proceeding to attack them again. They easily overpowered Yamamoto, Gokudera, Ken, and Chikusa, knocking them unconscious and leave only Mukuro and Tsuna to fight. Mukuro ordered M.M to wake up Fran as they were at disadvantage, but Fran fainted again once he saw the Vindices. Realizing that he had relying on Fran too much, he decided to take the matters alone, unleashing Genjuu Gagaia. However, unlike the previous attack, the Vindice are able to destroy every single crow.
As he and Tsuna both cornered, Chrome arrived as Reborn's sixth representative, offering to support Mukuro's illusion. Seeing Chrome's weak condition, Tsuna questioned why Mukuro didn't help providing her with his illusions. Mukuro refused to answer, telling him that it was none of his business which angered Tsuna who then asked did he abandoned Chrome, much to Mukuro's annoyance. But Reborn tell Tsuna that it wasn't Mukuro's fault but Chrome's own problem, revealing that Chrome cannot accept Mukuro's illusions due to her conflicting feelings between wanting to be acknowledged as her own person by Mukuro or keep living with Mukuro's illusionary organs. Reborn understood Mukuro's decision to leave Chrome in Namimori as he wanted to keep a distance between them since Chrome's heart has been struggling. Mukuro stated that he wanted Chrome to become an independent soldier who can stand up on her own because he saw great quality of a warrior in her and he would acknowledge her once more if she were to come against him as part of Team Reborn, something which Tsuna disagreed considering her feelings for Mukuro. Once Chrome finally able to found her resolve, she support his illusions, using full power of her flames. This impressed Mukuro as her flames are even stronger than Fran but then soon turned into shock when he realized that the tranquility of her flames is different from usual, feeling his heart was being healed. With their flames at the fullest power, he and Chrome unleashed a much powerful version of Genjuu Gagaia, Genjuu Mugaia, defeating two of three Vindice.
After Tsuna defeated Jack and being sucked into Bermuda's warp hole in attempt to prevent Reborn being taken away and the third day battle ended, he and his team except M.M and Fran returned to Kokuyo Land to treat their injuries. Shortly after, Tsuna visited Mukuro to discuss something with him. The next day, he and Kokuyo Gang members, along with other people who Tsuna had contacted yesterday night gathered at Tsuna's house. There, Tsuna requested for all of them to fight together. Mukuro was one of three people who offered to fight Jaeger, and like Xanxus, he threatened to cut off their alliance if Tsuna disagrees until Dino finally came up with a solution.
4th Day of Representative Battle of the Rainbow Edit
When Bermuda and Jaeger received report that Team Verde members they defeated were decoy puppets like other two teams, Jaeger suggested that they should go to where Big Pino and Small Gia are to prevent them being defeated one by one. However, Mukuro, Dino, Squalo, Xanxus, and Byakuran revealed themselves from their cover, preparing to fight them to prevent them from assisting their teammates. After Jaeger defeated Squalo, Byakuran, and Xanxus, Jaeger begins to attack Mukuro, but Mukuro able to defend himself just in time before Jaeger's attack could hit him, avoiding grave injuries. After Dino was defeated, he and Hibari fight together against Jaeger until Tsuna arrives. Finally realizing that Jaeger only able to short warped his full body twice, Mukuro and Hibari protected Tsuna, and Mukuro stopped Jaeger's movements by freezing his arms while the latter stabbed through their shoulders, giving Tsuna a chance to defeat Jaeger with converted X-Burner. Heavily injured, Mukuro and Hibari collapsed, unable to fight any longer. Before Tsuna could finish Jaeger, Bermuda interferes and released his curse and destroys the battler watches of the collapsed team members, disqualifying them and only leaving Tsuna and Reborn to fight Bermuda.
After Bermuda was defeated and Arcobaleno's curse are removed, Mukuro, Chrome, and Fran are submitted into Namimori Hospital along with the Varia, Hibari, Byakuran, Dino, and Basil, much to Tsuna's surprise. When Hibari was angered by the interruption caused by Xanxus and Byakuran, he destroyed the wall that led to Mukuro's room, making Mukuro and the other Kokuyo Gang members who are visiting them startled and he accidentally crushed his chocolate ice, which angered Mukuro and they started to fight, destroying some parts of the hospital. In the final chapter, Mukuro is last seen along with his gang when Tsuna narrates his thought of how now he has friends that he could rely on when it's necessary.