SSS-Rank Hero retiring to Start his Villain Arc

Leon lived an ordinary life like common folks in the modern world. Naive and normal he was, until he was sent to a different timeline, where he replaced Leon of that timeline. Not a modern world, but an ancient time for sure. The world was arcane for the boy, a world divided by classes of holy and commoners. Unfortunately, he wasn't a boy with a golden spoon in his mouth but rather a commoner like others. People in the holy class had the power akin to magic, but those in the lower class weren't that special. Like every story, Leon power aroused from slumber; an ordinary had awakened a power akin to what we call 'SSS LLevel.' When the authorities of Holy Palace found out, they enlisted him in their list of 'Hero'. Leon never cared. What he pondered all the time was to enjoy even this life. He wasn't transmigrated nor reincarenated. He ruminated about his purpose: 'why was he sent here?' at first, nothing surged in mind until his Heroic journey concluded; he had realized the purpose.