Hiro learned how skills worked in his month of grinding against goblins. They followed what became known as an evolutionary path. This was extremely similar to skill trees or tech trees in most RPGs or MMOs.
Only Main and Secondary Limit skills showed the evolution paths, however. The Secondary actives and passives did not. Hiro surmised that it probably stemmed from the latter needing different requirements and not just familiarity with the skill.
For example, one of Hiro's first skills Toughness [D] required one to experienced being beaten to the point of fainting to learn the skill. Most would think if you get beaten more and more it would evolve right?
Wrong, unlike Main and Limit skills, getting beaten even further would never increase [Toughness]. What allowed Hiro to evolve the skill came from mental resilience. Witnessing gruesome deaths, inhuman sights of battle, and cruelty gave him what he lacked.
