Not (Just) A Mage Lord Isekai - Chapter 58 - No Knowledge Without Cost

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

My name is Kaelith Yurei , and I am a first-year student at the Shin'ō Academy in the Soul Society. I have a friend who is destined to be quite famous in the Soul Society; his name is Sōsuke Aizen.Unlike this ambitious and idealistic friend, my goals are quite simple: through hard work and diligence, I aim to graduate and become a seated officer in the Gotei 13, from which point I will… I bounded with who!?Whenever Aizen enhances his strength, my power automatically synchronizes to match his level.Therefore, what I need to do from now on is very clear.“Sōsuke, you need to work hard. As long as you put in enough effort, I can live the life I desire!”

My brother* and I, from elementary school to university, had grown up together. We didn’t just get into the same degree, we’re also sharing the same dormitory room.