Xanxus was born and spent his early childhood in an impoverished region of Italy with only his mentally ill mother to care for him. His mother discovered his Flame of Wrath when he was very young and brought him to the Vongola Ninth. When she and Xanxus confronted the 9th and Xanxus demonstrated the Flame of Wrath, the 9th accepted Xanxus and placed his scarf around the boy, agreeing that Xanxus was indeed his son even though he had never seen the woman or her son before.
Eight years before the start of the Ring Conflict, Xanxus discovered his true heritage from Timoteo's diary, finding out he's not his real son, and Timoteo has no intention to give him the position of the next Vongola Boss because he doesn't have the Blood of Vongola. This made Xanxus feel betrayed, thinking that he was only adopted out of pity. Enraged by the revelation, Xanxus, along with his chosen Varia Guardians (minus Gola Moska), planned a rebellion which lead to the incident known as Cradle Affair. This rebellion, however, failed as Timoteo froze him with Vongola Primo's technique, Zero Point Breakthrough: First Edition. Xanxus was frozen for eight years, heavily guarded, until someone unfroze the ice and freed him.
After Squalo returned to Italy with the Half Vongola Rings, Xanxus was soon able to figure out that they were fakes and along with the rest of the Varia, set out for Japan. Once there, he was stopped by Iemitsu Sawada and agreed to hold the Vongola Tournament. After Tsunayoshi Sawada's intervention during the Lightning Ring Battle, Xanxus won the Sky Ring. Later during the Cloud Ring Battle, Xanxus programmed Gola Moska on a rampage until it was stopped by Tsuna; however, Xanxus had kidnapped Timoteo and placed him inside Moska. Xanxus used this as an excuse to fight Tsuna in the Sky Ring Battle. During the match, Xanxus eventually acquired all seven rings; however, the Sky Ring rejected him, as he wasn't a true Vongola descendent. Xanxus and his crew were then forced to resign.
In the Future, Shoichi revealed that the Vongola had launched a large scale international invasion led by the Varia against the Millefore Famiglia in Italy. The Varia, led by Xanxus, split into three groups to take over the Millefiore HQ to eliminate one of their Funeral Wreaths. During the invasion, Belphegor and Fran encountered one of the Six Funeral Wreaths, who was revealed to be Bel's brother, Rasiel, and his butler Olgert. Rasiel and Olgert quickly defeated the two. Olgert then sent three of his Heavy Rain Elephants to destroy the base that had been already taken over by the Varia. In the remains of the castle, an unharmed Xanxus sat on his throne relaxing. Rasiel, who was unimpressed by Xanxus, started to mock him by saying that he had been frozen by a fourteen-year-old, proceeding to call him lame. However, Xanxus ignored this and goaded Rasiel to attack him. Olgert sent one of his Heavy Rain Elephants to attack Xanxus, but only to have them be stopped in mid-flight and destroyed by Xanxus.
It is later revealed that Xanxus's Box Weapon, the Leone di Cieli, is the reason why their attacks kept on stopping. Rasiel, however, found this amusing and stated that Xanxus, who had a Sky attribute, was rejected by the Vongola Ring. Olgert, on the other hand, was worried that his Box Weapon had been petrified and then broken down, but Rasiel refused to listen to Olgert, and so the two attacked again only to see that after absorbing Xanxus' Flame of Wrath, the Liger Tempesta Di Cielo destroyed their Box Weapons. Xanxus told the two that he might let them choose the manner of their deaths if they brought Byakuran. However, Rasiel refused to listen to Xanxus' deal and attacked again with his Box Weapon only to find his Box Weapon slowly beginning to be petrified and starting to crumble. Olgert's body then slowly petrified, crumbled, and died.
Rasiel, who had realized that he couldn't win, started to beg that he would bring Byakuran and make Xanxus the leader of the combined Vongola with the Millefiore; however, unfortunately for Rasiel, Xanxus ignored his pleas and stated that he only wanted the Vongola at its strongest and that the Vongola needed Tsuna if it was going to be strong, proceeding to kill Rasiel. A hologram of Byakuran, who had seen the whole battle, then appeared and congratulated them and told everyone about Choice, and that the Vongola Rings will be on the line. Xanxus told Tsuna that he must show the Millefiore that the Vongola is the strongest.
At the last minute, the Varia rescued Lal Mirch, Gokudera, and Gamma from Bluebell and Zakuro, who were about to finish them off. Xanxus told Gokudera that the Varia had come to help them in the battle. Gokudera offered some advice about the Funeral Wreaths, but was only told to shut up by Xanxus.
Xanxus and the rest of the Varia then attacked the two Funeral Wreaths. Xanxus appeared to have killed Zakuro, but was attacked by Kikyo. The rest of the Real Six Funeral Wreaths finally believed that wiped out the Vongola; only to see that the people that they had fought were illusions created by Mukuro Rokudo, and then the Vongola attacked them. After Mukuro explains how he had escaped from Vindice Prison in a long conversation, the three Funeral Wreaths buy time to heal and to attack again. The two sides were about to attack when the last of the Real Six Furenal Wreaths, Ghost, appeared out of nowhere. The sudden appearance of Ghost shocked even Bluebell, Zakuro, and Kikyo while the rest of the Varia found their attacks on Ghost useless. Ghost then suddenly starts to drain everyone's Flames, friend or foe, and started to head to Uni's location, but fortunately Tsuna came and the two started absorbing each other's Flames, with Tsuna using the Zero Point Breakthrough: Revised to absorb Ghost, but the Flames, however, only go to Byakuran, who congratulated Tsuna for defeating Ghost so easily. Byakuran told everyone that he was so honored that the entire Vongola Famiglia was here and that he finds Tsunayoshi interesting for making Xanxus and Mukuro submit to him. Xanxus and Mukuro, who felt insulted by what Byakuran had said, attacked together, but found that their attacks didn't affect Byakuran. Byakuran states that everyone's Flames that were stolen by Ghost, have been transferred to him.
Tsuna told everyone that he will beat Byakuran, but he was no match for Byakuran and was overpowered. Tsuna, however, refuses to give in and unlocked his Vongola Ring's true form. After a fight, Byakuran was defeated by Tsuna. The Varia were about to kill Kikyo, but Shoichi told everyone that the Real Six Funeral Wreaths weren`t in any criminal records and that they were once normal humans. Kikyo says that they were kings and rulers in different worlds, but were cursed with misfortunes, and that Byakuran brought purpose into their lives. But before he can finish, Xanxus tells him to shut up, and used one of his X-Guns to apparently blow Kikyo's head off. Lussuria reassures everyone that he will keep him alive, the Varia then leave and head back to Italy. Xanxus' past counterpart is then seen receiving the memories about the future battle between Byakuran and Tsuna from Uni to prevent it from ever happening.
The rest of the Varia show up to the ceremony except for Xanxus; Squalo stated that Xanxus wouldn't show up because of Tsuna.
Xanxus is briefly seen predicting what Mammon's request was for; the Arcobaleno curse, and stated that he would decide if he would or wouldn't agree to the favor, depending on the situation. Xanxus is later revealed to have agreed to the favor to fight for Mammon, and was seen walking in Japan, observed by Dino. It is also revealed that he and the Varia are staying in the same hotel as the Cavallone Famiglia. Later, when Belphegor rushed into Xanxus's office and threw Tsuna inside, Xanxus was wearing a yukata and resting. Mammon informed him that Dino and Tsuna were Reborn's representatives, just like he had guessed. Xanxus then looks at Tsuna angrily and said that this time he would erase Tsuna once and for all. Suddenly, he sensed the presence of someone and shot his X-Guns at the ceiling; jumping down was a man who introduced himself as the planner of the "Representative Battle of the Rainbow", Wonomichi.
During the first battle of the Rainbow, Xanxus spends the time eating and sleeping, ignoring the conflict entirely. Mammon expresses their concern over Xanxus' participation in the Battle, but chooses to believe in Xanxus' power. During the second battle, when Kyoya Hibari arrives in their hotel room along with Fon, Mammon rushes to wake Xanxus after Hibari takes out Levi, Lussuria and Bel. Xanxus was awoken by the noise when Mammon came in, and ordered the Arcobaleno to find food for him. All that was left was Lussuria's shiruko, which Xanxus choked on. He followed Mammon into the common room to find water, since Bel had broken the water pipes. He sees the state of the battle with Fon and Hibari and intervenes by throwing the still-hot mochi at Squalo's head. Fon tries to take out Squalo and Xanxus at once, but only manages to break Squalo's watch. Mammon's intervention saves Xanxus' watch.
Xanxus attempts to kill Hibari, but Dino saves him at the last moment, in the name of Team Reborn. Xanxus reacts violently to Dino's jab at his loss to Tsuna in the Ring Conflict, and his old scars show themselves once more. He brings out Bester, who has gotten an upgrade from Talbot, the Armatura Platino. He uses Cambio Forma and fuses with Xanxus' X-Guns to create the Pistola Imperatore Animale. He and Hibari engage in battle, causing the entire top floor of the hotel to be blown away in the blast. It turns out that he changed the cartridge of his gun to protect his boss watch from Hibari's chain. The watch announces the end of the second battle, but Hibari was not satisfied by the result unless they end it properly, leading him to destroy his own boss watch, much to everyone's surprise. Watching this, Xanxus laughed and was about to destroy his boss watch as well, but was stopped by the other Varia members.
After the announcement of the result of the second day, his team was assaulted by a Vindice who came to destroy his boss watch. He and Squalo manage to protect the boss watch until Mammon came and the Vindice retreated, but were heavily injured as a result. Right at midnight, the watch announces the third battle, much to Mammon's and other Varia members' dismay. Shortly then, three Vindice appear in front of them. Mammon used their remaining present time to fool the Vindice and were able to escape from them. Not long after, Tsuna and Reborn returned from Vindice prison, Tsuna visited Xanxus and requested to lend him his strength. The next day, Xanxus and the other Varia members along with people who Tsuna had contacted yesterday night gathered in Tsuna's house and Tsuna requested them to fight together with him.
At the afternoon, during the fourth battle, Xanxus, Squalo, Dino, Mukuro, and Byakuran confront Jaeger and Bermuda, preventing them from leaving. As the battle starts, he and the others are shocked to witness Jaeger's short warp. Jaeger appeared behind Xanxus and cut off his right arm. Squalo attacked Jaeger in response, but was countered as Jaeger stabbed him through the chest, causing him to fall to the ground, unconscious, despite Xanxus' order. With Squalo unresponsive, Xanxus' anger rages, causing his old scars to appear. In a desperate attempt of protection, Mukuro summons a crystal barrier to protect the remaining members backs from Jaeger's short warp attacks, but even this does not stop Jaeger as he short warps his hand, straight in and through Byakuran's chest. As Byakuran falls, Xanxus moves to fire his weapon, but Jaeger evades and in a split second, slashes Xanxus' knee caps, causing him to fall to the ground as the rest of the Varia look on in horror.
Xanxus is eventually shown to be recuperating in a hospital while being fed by Lussuria and making Squalo run around. He gets into a fight with some of the other characters and brings out his guns; the fight causes some damage to the hospital facilities. Xanxus is last seen with the rest of the Varia when Tsuna thinks about the people he (Tsuna) can rely on, sitting slouched on his chair as usual.