Tsuna is often shot by Reborn with a bullet that increases his physical abilities in order for him to fulfill his regrets. After being informed that he is the heir to the Vongola Family, due in part to him being the great-great-great grandson of the first Vongola boss as well as the untimely deaths of the other candidates for the position, he is trained into becoming a mob-boss by the infant Vongola hitman Reborn. Because of his severely low self-esteem and lack of talent in any activity, Tsuna is known as 'No-Good Tsuna' at his middle school Namimori. Tsunayoshi Sawada, referred to as Tsuna, is the 14-year-old central character of the series. The story's shift into a darker and more serious tone by volume eight garnered even more praise, with some finding the characters being no longer simply used for slapstick comedy pleasing, and that the characters' evolving maturity is gratifying. Some attribute the story's humor to the peculiar characters, with one reviewer positively describing it is as an insane collection of characters. Various publications that review manga series have mostly praised the eccentric cast of characters in Reborn!. The characters transported from the past into this future are joined by adult versions of the ones who have yet to be replaced with their younger selves. When the story later shifts into an alternate future, nine years and ten months later, in which some of the characters are transported into, they have to adapt to a world where every Vongola member, as well as their allies, is being hunted down by a powerful rival family of that era, the Millefiore.

Enemies such as the Kokuyo Gang, a group of Mafia criminals, and The Varia, an elite team of assassins, prove to be formidable challenges for Tsuna's new 'family'. Though they encounter new allies in the form of other hitmen and mafiosi, they also attract attention from those who want to annihilate the future boss of the Vongola Family. Tsuna gradually becomes a more confident person and leader, while Reborn slowly recruits members into Tsuna's own Mafia family, which mostly includes Tsuna's schoolmates. The current Vongola boss thus sends his most trusted member, the titular character and infant hitman, Reborn, to train the future boss. The story centers around middle school student Tsunayoshi 'Tsuna' Sawada, who finds out that he is the heir to the most powerful Mafia family, the Vongola.

However, their connection to an Italian Mafia family results in a majority of other characters introduced being mainly of Italian descent, including most of its antagonists.

Set in a fictional town called Namimori in modern Japan, the series' main protagonists are of Japanese descent. The characters of the Reborn! anime and manga series, known in Japan as Katekyō Hitman Reborn!, are created by Akira Amano.

Katekyo hitman reborn opening 5