Rebirth in One Piece as Deputy Captain - Chapter 544

Imperial soldier Suldak Helansa joined the infantry regiment of the Grand Duke Newman in Bena province, participated in the Warsaw plane war, and rescued a young man in the forest battle. This qualified young man suffered some injuries. Injured, still a mute, Suldak brought him into the barracks, and what we are telling is the story of this young man of unknown origin…

- Description from novelbuddy

The strongest Hero in history, Nakamoto Keisuke.

He was summoned to another world in the spring at the age of fifteen years old, and when he successfully defeated The Demon King, he was already thirty-two years old. He had finished his role in another world and returned to Japan, but the reality that had been waiting for him was, “Middle school graduate with seventeen years of blank period” label. He couldn’t find a decent job and had no choice but to spend his days working part-time.

Before such a Nakamoto, a beautiful girl who self-proclaimed had come to return the favor, appeared. “I’m the reward for Hero who had defeated Demon King”, said Angelica, a girl who came from another world that persistently tempted Nakamoto to make her his wife. Together with a high school girl who looks like Elsa, his beloved girl that he lost in another world, Nakamoto with his cheats skills got caught in a harem life.

Qin Fen had once been abandoned and broken until, one day, he picked up a liquid metal ball that turned his life upside down.
This is the interplanetary era where Classical and Modern Wushu are competing. Classical Wushu enables followers to grow as powerful as dragons and elephants, whereas Modern Wushu enables followers to freeze monsters or even extinguish erupting volcanoes.
Qin Fen steps onto the stage of both Classical and Modern Wushu while belonging to neither. With the exquisite combination of both Classical Wushu and modern technology, he will pave a way to become a legend.
He is one of a kind, a favored son of heaven.