Leo finally became a Gold Rank mercenary after finishing ten Silver Rank missions. He realized that ranking up was fairly easy, but no one abused it because the missions became increasingly hard when they ranked up.
When he checked out the missions in the Gold Rank, he finally saw that there was some difficulty associated with them. For the first time, he would have to put real effort into the missions.
This was because the missions now involved dealing with 3rd Circle Magical Beasts. However, with the increase in his strength, it wouldn't pose any danger to him.
Along with his increase in Mercenary Rank, he had one major breakthrough. He was able to become a Middle-Stage 3rd Star Mage. This itself increased his strength by 40%. He was also incredibly close to increasing his proficiency in Valkyr's Deception from Intermediate to Advanced Proficiency.