Middle school student Tsuna has always gone through life in last place; but when he learns that he is getting a tutor to help him raise his game, he had no idea that his benefactor would turn out to be... an infant and a mafia hitman?! As if things can't get much stranger, Tsuna then discovers that the powerful Vongola family has chosen him to be its next successor - a position that has control over all mafia families. Joined by friends Tsuna unwillingly starts out on his path to becoming a mafia boss - if he can survive the training!
Both of those shounen battle mangas start with almost identical setting. The main hero is a weak, bullied and cowardly kid who suddenly learns, about his amazing lineage and mysterious power he can inherit ( mafia family Vongola in Hitman Reborn, strongest magical king in Hyde and Closer).
Due to this sudden development many assasisns targets young boys live. To keep him alive he receives a guardian in childish body (Teddybear Hyde in Hyde and closer and child Reborn in Hitman Reborn). This guardian, despite his appearance, is powerful being and skilled teacher, who will direct young boy towards his powers.