Template:Sky Box Animals Natsu, Tsuna's exclusive Vongola Box Weapon, was created by remodeling a Sky-attribute Sky Lion. He is imbued with the Vongola's latest technology. He is still in the process of growth and Tsuna has named him Natsu. His figure is that of a small, immature lion that acts like its master in and out of battle, going from a serious and powerful animal to a timid and easily frightened cub. As they are both based on the same Box Animal, Natsu has some similarities to Xanxus's Bester.
Attacks and Abilities just like the normal Leone di Cielo, it can assimilate the enemy with the battlefield through the Harmony of its Flames. Its greatest feature is transforming to the same weapons used by Vongola Primo, via Cambio Forma.
- Ruggito di Cieli: Like the Sky Lion, Natsu sends Sky Flames carrying the Harmony property at the opponents through a roar and they are then assimilated with the surrounding battlefield. It was first used to petrify Torikabuto's Rete Serpente di Mare.
- Cambio Forma
- Modo Difesa: Mantello di Vongola Primo (Defense Mode: Vongola First's Cloak): Natsu changes into a long, black Cloak covered in Sky Flames that has the capacity to nullify attacks. Tsuna wears the cloak over his shoulder, which is attached to his left X-Glove through a metal piece with the Roman Numeral "I" on it. After Tsuna attained the original Sky Vongola Ring, the metal piece changed shape and the cloak's powers greatly improved.
- Modo Atacco: Mitena di Vongola Primo Template:ImgRight(Attack Mode: Vongola First's Gauntlet): In this form, Natsu combines with Tsuna's right X-Glove and becomes a metal gauntlet with the Roman Numeral "I" on it. He can then concentrate its Flames in his fist to deliver a punching technique called "Big Bang Axel", which is said to rival the X-Burner in power. After Tsuna attained the original Sky Vongola Ring, the gauntlet changed shape, becoming longer and gaining stylizing spikes, and the technique gained even more power, being renamed "Burning Axel".
- It was first opened incorrectly and appeared in a monstrous shape.
- In the Reborn! Fandom, his number is 72, the reverse of Tsuna's 27.
- The katakana for Natsu, "ナツ," is the reverse of Tsuna's name in katakana.
- His eyes are similar to Tsuna's in Hyper Dying Will Mode.
- When Reborn wanted a pet for Tsuna, he had suggested a lion.
- During Future Final Battle Arc's end, Tsuna and his family couldn't take the box weapons with them because they didn't exist in the past. Thanks to Verde, Natsu became a Vongola Box Ring at first. With the help of Talbot, it later became one with the Vongola Sky Ring and at the end also became a Ring of the Sky Version X.
- There have been some translations of "Natsu" from Japanese to English, into the word "Nuts".
- Natsu gets along with Enma, letting Enma pet him and play with him; also Natsu, at first, wouldn't attack Enma until Enma's monstrous form was revealed to him, thus scaring him.
- Since Natsu is timid and scared easily out of battle, Uri, Gokudera's box weapon, attacks Natsu when it won't practice battle positioning. In turn, Natsu and Uri dragged Tsuna into the fight and Gokudera yelled at Uri.
- Natsu can also sense Tsuna's feelings.