Daniel is one of the main antagonists in Kai's storyline. There are a few antagonists, but Daniel was certainly a large threat. Daniel was a naga that was gifted by the gods by conjoining his soul with that of an DemiGod. The soul of a DemiGod was so pure that the soul of the mortal naga ended up beginning to corrupt him, effectively driving them both into madness. Eventually they started a rebellion, driving the nagas and the gods against all else for a false righteous cause.

Daniel ends up killing Coal in combat, and in a fit of rage and sorrow, Kaito in turn kills Daniel and causes the entire field of combat to distort, thus ending the battle for good. After a year of toil and growing madness, Kaito resurrects his lost love, but at the price of his own leg which was used in the revival process to repair Coal's damaged bodily tissues. Which is why Kaito has the mechanical leg later on in life.