One Piece: Unreal Adventure - Chapter 10: Fighting Crabs?

Ruins stretched across the landscape in the apocalypse after the nuclear war.

【Hilarious and Adorable Novel】As a military doctor with expertise in both Chinese and Western medicine, Gu Changsheng felt that this time-traveling journey was ridiculously screwy!Once he traveled back, he was gifted a son, hmm, it felt quite nice.Having a son to rely on in old age, plus owning a steady estate, wealth and glory weren't hard to come by.A lousy father turns over a new leaf? Sorry, mom here isn't fond of revisiting the past!Son, what kind of dad do you want? From the multitude of handsome and wealthy young men out there, let’s pick and choose…This is the story of a bizarrely outstanding female doctor, strutting through ancient times with her child in tow!

Kusumi Chiharu gets a message one day that he has been selected as a Monster Tamer, and he is prompted to select his first partner. He thinks little of the message until he receives a giant package the following day. He assumes it’s the porn books he ordered, but the huge box ends up containing the unconscious body of a near-naked girl! She’s a ghoul named Zonmi, who intends to fight beside him as his partner.

However, it turns out that other hot girls at his mostly-male high school are also magical entities, and Zonmi may not be the only one who wants to fight by his side!

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