Uni inherited the Orange Arcobaleno Pacifier from her mother, and this Pacifier has been passed down in their Famiglia for three generations. When Aria passed away, Uni learns of the Mafia, and the leadership role she must play for the Giglio Nero Famiglia. Uni was originally kind-hearted and caring towards others, and during that time, she came to know Reborn, whom she knows as her uncle. The Giglio Nero Famiglia was losing strength and power. In order to keep the Famiglia's power and avoid any of her Famiglia members hurt even further, she meets Byakuran, who schedules a meeting with her. After the meeting, Uni emerges from Byakuran's office with a completely different attitude and personality (according to Shoichi, her heart was dulled with a powerful drug). She then announces that the two Famiglias will merge and become the Millefiore Famiglia, in which Byakuran would be the head. As a result of the merge, she has become bitter and cold-hearted.
She was the head of the Black Spell and Captain of the 1st Calendula Squad. After the first round of Choice, she had recovered from the drug and intervened to say that the promise that Shoichi said that they made is valid, and Byakuran should honor it. When Byakuran ignored her suggestion, she decided to leave the Millefiore Famiglia and asked Tsuna to protect her.
In the Future, she ranks as the Millefiore Famiglia's overall second-in-command and the leader of the Black Spell Division.
After the Choice battle in which Vongola team lost, Shoichi Irie demands a rematch based on the fact that Shoichi won the last game of Choice that he played in university after which Byakuran said that Shoichi was allowed to make any condition in their next match. Byakuran refused, claiming not to remember the conversation. Uni then shows up claiming that as co-head of the Millefiore Famiglia, she should have a vote in the matter. She reveals that she is no longer under the influence of the drugs that Byakuran had given and that she had protected her heart "by going to a faraway place". When Byakuran refuses her request she breaks of the pact between the Giglio Nero and the Gesso Famiglias. She then asks Tsuna to protect her while showing him that she has taken back the Arcobaleno Pacifiers.
When Byakuran tries telling her to return them she states that what Byakuran has cannot be called Tri-Ni-Set because if the hearts of the owners are dark the Pacifiers will not show their true shine. She then proceeds to restore them with her power, which triggers Byakuran anger and greed, prompting him to attack her and to force her back only to be stopped by Reborn's bullet who states that no matter the circumstances he cannot allow him to harm the leader of the Arcobaleno who he also states is the granddaughter of an acquaintance of his.
A few moments later, Byakuran proposes to Uni that the Vongola Rings stay with the Vongola Famiglia in exchange for her return to the Millefiore side together with the Arcobaleno Pacifiers. But Uni refuses the proposal saying that she knows why Byakuran wants her back. She also adds that Byakuran does not have any right to the Tri-Ni-Set since the Vongola Rings and Arcobaleno Pacifiers are properties of the Vongola and Arcobaleno respectively. She swears that as long as her soul exists, as the Sky Arcobaleno that carries the parts of the Tri-Ni-Set, she will not allow any scramble for the Tri-Ni-Set. Moreover, she invalidates the Choice Game.
In response, Byakuran threatens Uni that harm will befall the members of the Black Spell if she continues with the betrayal. But Uni stays firm with her decision and once Tsuna got resolved with helping her, she proceeds to escape back to Namimori with the Vongola Famiglia.
After she visited Lal Mirch in her hospital room and asked her to take care all of Arcobaleno Pacifiers, she along with Tsuna and the others return to the Past and summon Vongola Primo through Tsuna's Vongola Ring. Her real identity was kept as secret so it won't change the Future. Vongola Primo appears, answering Uni's will and accepting her condition which is still unknown what kind of condition she given to him so he can help Tsuna. She then staying at Tsuna's house for a while and become his tutor after receiving the Sky Pacifier from Aria in the past. When Reborn told her that she might meet her in this time, she refused saying it’s okay and that this is not the time they should meet and Aria knows of this.
Later, without Tsuna's knowledge, she transferred to Namimori Junior High School, as she was his tutor. She was seen at all succession tests but didn't do anything but watch. That is, until Demon Spade's succession test. Uni, along with the other girls were trapped inside a field in Kokuyo Land by Demon as hostages. As it gets more and more difficult to breathe, Uni uses her power to ease the breathing. After a while she gets exhausted, but was able to guide Reborn and the other Arcobaleno to her location. She watches both Tsuna and Chrome get their inheritance. As she prepares to leave, she thanks Tsuna's mother and the rest for their hospitality and goes back to the future with the rest to face Byakuran and the Six Funeral Wreaths.
At Kawahira's residence, Uni was kidnapped by Torikabuto who was disguised as Lambo but later was defeated by Tsuna with the help of Chrome's Vongola Box Weapon. Uni is later seen at the final showdown between Millefiore and Vongola. Uni was summoned by the resonance of 2 strong Sky Flames into the battlefield where Byakuran and Tsuna were fighting. Uni later sacrificed herself to revive the dead Arcobaleno by using her Flames, but it grew weaker because she feared death. Gamma later got inside the barrier due to Basil's Box Weapon and coax Uni not to be frightened of death since Gamma will accompany her together. Uni and Gamma then became the sacrifice to revive the Arcobaleno.
Uni, before dying, told a few people from the past about Tsuna and the Guardians' battles in the Future using the Flame of Life.
Future Uni emerges from the forest as the Sky Arcobaleno pacifier becomes inflamed, showing that the successor had arrived. She had chosen Byakuran, the Six Funeral Wreaths, Gamma, Nosaru, and Tazaru to be her representatives for the Representative Battle of the Rainbow. Later, Uni is seen telling Byakuran and Gamma that they had some planning to do after Wonomichi explains the rules of the upcoming battle. When Byakuran arrives in Japan and offers to Tsuna that they form an alliance, the former reveals that Uni was on some "business" before the battle.
Later, Uni appears with Reborn and the others. Reborn asks Uni if Byakuran and the others were all right, to which she responded that Shoichi and Deisy were tending to their wounds with their Sun Flame's Activation characteristic. Reborn asks Uni if she foretold the current situation, the latter replying in the positive. Uni explains that she had known about Bermuda and his anger towards the Arcobaleno, but didn't understand what the anger signified. Colonnello asks Uni if she could tell any more of the future, but Uni responds that she only saw Checker Face's face, and that each time she saw it, her heart hurt as if it was being gouged out. Reborn, however, hands Uni a handkerchief and tells her not to worry too much, stating that he didn't like seeing her like that. Uni tearfully thanks Reborn.
After the third day of battle, the Arcobaleno (sans Skull ) meet with Reborn. When Reborn gets into an argument with Colonnello, Verde, and Mammon, Mammon accidentally hits Uni with an illusionary attack. This ended the fight, as the Arcobaleno parted ways immediately afterwards.