The blue glow on Kadi's body is Kadi's power.
Having prepared for so long and endured so many attacks, Kadi had now become accustomed to it. It's inseparable from his unique physique; he was inherently different from normal people. This blue skin was actually a manifestation of his extraordinary ability.
His blue skin contained the power of devouring, a power that the Soul Devourers had been pursuing. This power inherently possesses a strong devouring nature, but to Kadi, it exists in another form.
Long ago, Kadi couldn't understand what this blue energy truly was. Later, through adventures with Igor, he gradually came to understand his special nature. He also learned some secrets about himself. As for where he came from, he seemed to have some clues. However, these questions could only be answered once he overcame the current difficulties.
For now, he needs to reason with this Water Spirit.
