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Drake felt like he was comprehending something, enlightened by something much deeper, his endless perseverance, and his own motivation to never give up, even if he's torn to shreds, changed something within him.
Every Soul that is reincarnated and obtain a Unique Skill receives this Unique Skill inside of their very Souls, the reason why Greenwood is able to extract them from people to begin with is because he extracts the souls and crystalize them.
With that, he's able to either add them into equipment or even absorb them into a physical, flesh body… However, for that very reason, that Unique Skills remain within one's soul.
Is that when someone's soul evolves and becomes stronger, the Unique Skills tend to change and become stronger as well.