Uni is the third Sky of the last generation of Arcobaleno, daughter of Aria, and the granddaughter of Luce. She was the Millefiore Famiglia's former second-in-command and is the current boss of the Giglio Nero Famiglia as a result of Aria's early death.
Uni wears the Giglio Nero clothing that her predecessors wore: a big, white, mushroom-shaped hat with half of the emblem of the Giglio Nero on it and a large black bar at the sides, with two orange stripes flanking it. Her Giglio Nero clothing also consists of a long coat with the Giglio Nero emblem at the bottom right and left sides. Underneath, she wears a pant-skirt and a short blouse that reveals her stomach. When in her Millefiore uniform, Uni dons a black jacket to symbolize that she is part of the Black Spell. However, she also wears a White Spell Millefiore coat over it. She has the appearance of being around 10-years-old.
After the Future Arc, when Uni returns, she wears a white uniform that is similar to the Millefiore one.
She has blue eyes and her hair is short in front, but longer at the back. In the manga color spreads, Uni's hair is a medium to dark blue like her eyes; however, her anime appearance dons a darker hair color of dark teal in nature.
Uni is a kind-hearted and polite girl, thanking people multiple times for helping her. Uni is also unselfish, willing to sacrifice her own life in the future to revive the dead Arcobaleno to restore balance to the Tri-ni-sette. She's also accepting and forgiving, harboring no ill will towards Byakuran despite everything his future self had done in the future.
Uni has the power to read people's mind and feelings; it has also been said that she knew what would happen to herself before she met with Byakuran. This Ability seems to have expanded to the Ability to see the Future, although that Ability has been growing weaker due to the curse she's bearing. She reveals that, like Byakuran, she also possesses the Ability to share her mind with the alternate versions of herself from different parallel worlds. She, like the other Sky Arcobalenos, Aria and Luce, has the power to revive or reconstruct her fellow Arcobaleno who have died temporarily at cost of her own life. It also appears as though she was the one who willed the Vongola Rings to bring back Vongola Primo during the Primo Family Arc. Additionally, the Tri-Ni-Set seems to respond to Uni's wishes, most likely because she is the boss of the Arcobaleno. Byakuran stated that the Sky Arcobaleno has authority over the Tri-Ni-Set. During the Inheritance Ceremony Arc, Timoteo reveals that he knows everything that happened in the future that Uni sent him through his dreams.
- Uni, in the future, held all the Arcobaleno Pacifiers except for Colonnello's, which was possessed by Lal Mirch.
- Like her mother and grandmother, she has the emblem of the Giglio Nero tattooed under her left eye. Her ancestor has the same tattoo but under the right eye instead of the left. And the symbol itself is based on the (presumably) Sky stone of the original Tri-ni-sette.
- She has a character song called Kokoro no Hoshi.
- The character bio for Uni on the official Reborn anime website mentions that Uni appears to be about 10 years old; however, her age has not been dead set.