The supporting character's harem is very normal - Chapter 409 Dorack returns

Asura – proud, warlike, cruel, heartless.

A young adult with a mysterious background follows his father’s prophecy and enters the world of «Rebirth», somehow managing to inherit the Xuanyuan Sword, and also unexpectedly obtaining the hidden class Asura.

Following the tracks of both virtual and reality he gradually discovers a secret hidden for the past billion years.

Asura, the class that used to shake the lands has now completely disappeared. He must now go on a journey to find the true meaning of Asura—until the day he transforms and becomes one with Asura.

- Description from Novelupdates

A man who was once called the world’s strongest agent ended up becoming a teacher after his retirement, to train a new generation of agents. After many years of training his disciples, he was killed at the age of 60 by a secret organization’s ruse and was reincarnated in another world with his memories intact. Though he was surprised by the existence of magic and the strange species in that world, he quickly adapted to his condition as a newborn and took advantage of it. He acquired special magic and gained a massive amount of strength thanks to his rigorous discipline, in order to reach his goal: Resume his career as a teacher which he left halfway through in his previous life. This is the story of a man, who, based on the memories and the experiences of his previous life, became a teacher who travels through the world with his students.