MTL - One Piece: The Long Road - Chapter 13 Step 1 of Thinking Stronger

Damian's life is bland, miserable and getting worse. After hitting rock bottom, he travel to another world. In an alternate Albion, he faces dangers and delights beyond anything he could have dreamed. The new and powerful sorcerer soon realises it will be a struggle to survive.

300 years after becoming a Demon King, the war angel Ansel faced a serious problem during a certain expedition.

——His halo grew out once again.

Ansel picked an auspicious place that was definitely and absolutely unmanned, and while cursing, he simply and skillfully broke off the newly emerged angelic aperture.

This thing was growing out faster and faster!

When will his undercover mission end already?!

The demon realm was divided and ruled by three kings. Demon King Saros was forced to go on an expedition with his sworn enemy, Ansel.

One day, he noticed Ansel sneaking away from everyone.

Saros decisively followed him, and was shocked to find that his sworn enemy, another demon king of the demon realm, took off a horn headband and fake bat wings. As Ansel cursed up and down, Saros watched as he skillfully removed a cute halo from his head.

This irritable, easily angered guy who loves fighting; the type that was cute when he shuts up but could make even demons angered to the point of ascending to heaven the moment he opens his mouth…

..was actually an angel???

- 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.

As a reward for preventing the assassination of the current king, Rou Coral is rewarded with becoming and escort of the Royal Princess Reaina who the boy adores so much. Thus begins his new life of servitude under his beloved high-headed Princess full of happenings and happy little accidents. Surrounded with Busty Head Maid, young Loli Princess and a Childhood Friend Maid, for the boy knight today is yet another day of struggle in this womanly paradise!!