Ding!
Leon was excited about the final reward, and so the panel appeared before him.
[The E-Level skill 'Fire Breath' has been upgraded to B-Level Skill 'Dragon Flames'!]
Much like with talents and many other things, skills could also be classified among several categories: F, E, D, C, B, A, S, SS, SSS.
There was no W ranking this time, ending at SSS, which was still pretty damn powerful.
Most skills, even legendary ones, usually end up being at A or S-level at the start, but as you keep upgrading them using skill points, then there is a chance for them to level up.
Skill points can be used to upgrade a skill's level, but note that skill level does NOT refer to its type.
For example, a level 2 'Fire Breath' doesn't mean that it would change from its E ranking to something better.
The only way to make a skill rank up is to: either buy an item for that, though it cost a fortune.
Or… to complete an advancement quest.