Why is Background Character the Strongest Now? - Chapter 20

Xia Yan passed through to the world of Bleach and became an eleven-year-old boy. He was rescued by Renji and Rukia in the Seireitei, and he was studying at Shin’ō Academy, and he was a student of the same class.
He is a top student of the Shin’ō Academy, and he is vying for strength by various teams. In the end, he has entered the first team of secret mobile maneuvers.
He will use punishment to kill every person who tries to threaten the martyrdom and protect his loved one.

- Description from Novelupdates

“I'm sorry, Kiara,” he began, his voice heavy with regret. “We regret to inform you that we can no longer administer these drugs to you.”I was dying. I was going to die.Kiara Williams was dying from Stomach Cancer. With only a few months to live, she could only stay at the hospital.Alone. Her annoying husband did not come and visit her, only her best friend, Jessica remained by her side. She could no longer pay the bills and was sent home.Arriving at her house she caught Jessica and her husband together in bed. In a fight, her husband pushed Kiara down the stairs.Kiara died.Or did she? She was reborn ten years back, but rather than being destroyed, she begged a strange man for a one night stand unless she may die again.

Naofumi Iwatani, an uncharismatic Otaku who spends his days on games and manga, suddenly finds himself summoned to a parallel universe! He discovers he is one of four heroes equipped with legendary weapons and tasked with saving the world from its prophesied destruction.

As the Shield Hero, the weakest of the heroes, all is not as it seems. Naofumi is soon alone, penniless, and betrayed. With no one to turn to, and nowhere to run, he is left with only his shield. Now, Naofumi must rise to become the legendary Shield Hero and save the world!