Otherworldly Munchkin: Let's Speedrun the Dungeon with Only 1 HP! #1

Plot

16-year-old Yukito Kirihara lives with his 14-year-old sister Sana, who he treasures more than anyone else. But when an ogre shows up outside of his window looking for Sana (a shocking event in its own right), Yukito leaps to save her—and is killed. He's offered the promise of reincarnation by a mysterious woman, who gives him a character sheet in a mysterious book. But when Yukito sees Sana in there, too, he sets off to find her, and save them both—even if he only has 1 hp with which to do it!