[Reborn!] The Ultimate Sun

From childhood, Ryohei Sasagawa knew he had to give everything he had to protect his one and only little sister, Kyoko Sasagawa. When he was twenty-five, Ryohei really did protect Kyoko at the cost of his own life. But... "Why did I open my eyes and find myself twenty years in the past? This is extremely unscientific!" This proves that the word "science" actually exists somewhere in Ryohei Sasagawa's brain. In short, Big Brother goes back from twenty years in the future to when he was five, only to discover that his sister is just three. He resolves to protect Kyoko with even more determination. And then... Who knows?