Guide to Failing as a Full-Level Green Tea - Chapter 288.1 - The Old Fox

I was reincarnated into another world, and I’ve brought the Horadric Cube from Diablo II with me.
Being a knight is cool and all, but being a wizard is pretty neat as well.
So, which one will it be?

A girl who calls herself a demon appeared before a timid student named Fumio Kijima. The girl claims to be a campaign girl from the demon world and she gives him the ability to “summon a room” from nothing but thin air.

At first he doesn’t abuse his power and uses it as a convenient ability in his every day life. But one day he is bullied by his classmates under a false pretext and is made to suffer. In the past he would have wallowed in misery and cried himself to sleep, but now he had a power to call upon.

“I will make them regret it!”

Using his powers to summon his “room”, he begins his era of confinement and brainwashing to achieve his goals.

When Gao Ge is awake, he finds that he has returned to the earth before aura reappeared. The world will usher in an era of practitioners soon. Various schools of cultivation and families of ancient martial arts will spring up. As a man who is living a second life, Gao Ge will never let go of the chance to grasp the resources, cultivation methods and treasure places first.

In this era, there are many geniuses and I, Gao Ge, shall be the king of them.

- Description from Novelupdates

The main character is reborn as a character from a game he played in his previous life. This character falls into ruin along with the villainess, spending the last of their years together. A man named Kururi Helan.