Yukito Kirihara

At 16 years old, Kirihara Yukito is a shut-in after dropping out from school due to bullying. Spending his days locked away in his home playing video games with restrictions to add a layer of challenge. His only friend and supporter during this time is his younger sister, Kirihara Sana. One day, he and his sister are attacked by a mysterious hulking monstrosity, with Yukito dying instantly due to a slash to the back.

This isn't the end of Yukito's story, however.

Brought back to life by a mysterious goddess known only as Nephilia. Nephilia is quick to inform him that he has died, though she reassures him that his sister is safe. She introduces him to an artifact of great power in her possession, known only as The World Book, stating that with it, she would decide his fate in the Everworld. Looking away for a moment to prepare an article of Yukito's equipment, she doesn't notice as Yukito taking a look through the contents of the book. Yukito is shocked to find that an entry for his sister is within its pages, signifying that Sana had also perished at the hand of the monster, and is somewhere in Everworld. Appalled at Nephilia's lie and at the fact that Sana may well be in danger, he lashes out at the goddess, bringing a dice-like artifact tumbling to the ground, shattering the floor beneath his feet and sending both him and the World Book into the Everworld.

Starting his journey as a Level 1 wizard, he inspects the World Book, finding it full of information of his environment and the inhabitants of the world around him. Along with this information, he realizes that his own entry reveals that he only has a single Hit Point, meaning even a small attack from an enemy could kill him. He ventures forward with two goals: to survive and to find his sister, stopping at nothing to accomplish both.