Giannini is the son of Giannichi from the Vongola Famiglia that focuses on engineering. Weapon tuning, modification, and creation is no problem for Giannini's family because they have been supplying the Vongola with specialized Weapons for many generations. Giannini gets really fired up when it comes to his work and loves working on rare items. He is said to have the same traits in his work like his father. He seems to travel using a Round Machine, but he doesn't need to in the Future Arc.
He is quite short and rotund, with short black hair, a shiny complexion and large eyes. He has grown somewhat taller in the future. Like most other Vongola members, he usually wears a plain dark suit.
Ordered by the 9th, Giannini leaves Italy to tune up overseas Famiglia Members' weapons. He is well approved by Reborn because he is the son of Giannichi. Reborn seems to know Giannini's father, and it is also revealed that Giannichi has helped Reborn in some way. Giannini says himself that he isn't on the same level as his father. He is given Gokudera's Bombs, Lambo's Ten Year Bazooka and Reborn gives him Leon to tune up. He turns Gokudera's Bombs into a magic trick shooting out doves when exploded. Reborn tries to shoot Tsuna, but the power in Leon was so greatly reduced that it didn't go forward an inch and the Ten Year Bazooka just shrinks Gokudera so that he looks like a child.
Giannini is the lead Engineer in the Vongola Famiglia in the future and is also the one who built the entire underground base upon Future Tsuna's request. His skills have improved greatly from before the Kokuyo Arc, as though he still has some errors in his works, he can easily fix them right away. It appears that Fuuta has been taken under his wing. He has also grown taller. When Haru and Kyoko went on strike, he and Reborn crossdressed to show support for them.
- He seems to have a rivalry with Irie Shoichi and Spanner, trying to prove he's the best mechanic for the Vongola. However, he was afraid of being chosen as a mechanic for Choice.
- He was featured in Yakusoku no Basho e - vs. Millefiore.