One Piece: My Crew is from Other World - Chapter 146: The Advancement of the Plan

In this world, orphans or just anyone without any hope for this world can apply to become a “walking target”. Their sole purpose is to be a practicing dummy for the students of the sect who train to become “Demon Hunters”. These students need “walking targets” to test their training on living, walking beings of flesh and blood.
In exchange, they will be provided with food & shelter.
Their existence value is lower than a slave, but if they manage to survive a few years they can receive a life long pension.

Xiao Han is such a “walking target”. One time he witnessed a powerful Demon Hunter disposing a demon. When the demon died, a fragment of the demon that managed to escape entered Xiao Han’s heart. No one saw that, and even Xiao Han himself only felt some discomfort for an instant.

But that thing in his heart soon gave Xiao Han visions, and the first one is of a cultivation method which allows Xiao Han to absorb damage to enhance his body. A perfect cultivation method for a “walking target”.

- Description from Novelupdates

He is a firefighter, his face blackened by smoke, coming from the flames, and in one glance, his life was set.The first time he met the young girl, she bent slightly towards him and said, “Sir...”He was amused, thinking she was a little eccentric...She had never seen such a handsome man, tall like a general, and when their eyes met, she was bewildered and forgot she wasn't in the palace...Later, during the school art performance, the young girl sat on stage, dressed in ancient costume, with a veil covering her face, holding a pipa, her eyebrows shy, appearing both delicate and graceful...He raised his alluring eyes, recalling the girl said she knew a little about musical instruments and chess; she was modest.Seeing him in the audience, her heart raced, wanting to perform well but accidentally struck the wrong chord, fortunately unnoticed by him...Later still, the young girl, now grown, suddenly confessed to him, and the usually calm and composed man was caught off guard...It was a love she had hidden deep in her heart for many years, like shards of glass in her palm, both a patient endurance and impulsive agony...The joy of love at first sight, a hidden affection...She would never know that the brother she secretly admired had been waiting for her to grow up...

After 2000 years has passed, the ruthless demon lord has just been reincarnated! But his aptitude at an academy for nurturing candidates for demon lords is, “Inept”!?

Having the capability to destroy humans, elementals, and gods, after a long period of countless wars and strife, Arnos the demon lord became sick and tired of all that and longed for a peaceful world, so he decided to reincarnate to the future.

However, what awaited him after his reincarnation is a world too used to peace that his descendants became too weak due to a huge weakening in magical powers.

Arnos enrolled in the Maou Academy established to find any students that might be the demon lord’s reincarnation, but his powers are too extraordinary that those in the academy couldn’t properly judge his powers, so he’s regarded as an “Inept student”. Being underestimated and avoided by most people there, he recruited a girl friendly to him, Misha, as his subordinate and comrade, and strives to climb to the top of the demon race hierarchy to one day reclaim his former title and status.

The world is about to be destroyed to make way for an interstellar transmission array formation. A young man journeys through the stars to save his home. A bloody fantasy adventure.