Uni is the daughter of Aria and granddaughter of Luce. She is a young girl who is the Millefiore Family's second-in-command and the former leader of the Giglio Nero Famiglia as a result of the early death of her mother, whom she did not know was a Mafia boss. She inherited the Orange Arcobaleno Pacifier from her mother. This Pacifier has been passed down in their family for generations. When Aria passed away, she learns of the mafia and the leadership role she must play for the Giglio Nero Family. Uni was originally kindhearted and caring towards others and during that time she came to know Reborn whom she knows as her Grandfather. The Giglio Nero Family was losing strength and power. In order to keep the family's power, 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 (Acording to Shoichi her heart was dulled with a powerful drug). She then announces that the two families will merge and become the Millefiore Family. As a result of the merge, she has become bitter, and cold hearted. She is now the head of the Black Spell and captain of the 1st Calendula Squad. After the first round of Choice, she 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 family and asked Tsuna to protect her.
SpoilerIt has been speculated that Uni has the power to either read someone's mind or read their feelings, it has also been said that she knew what would happen to herself before she met with Byakuran.
In chapter 254 of the manga she reveals that like Byakuran she also posseses the ability to share her mind with the alternate versions of herself.