(Spoiler)
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The novel did not say anything about that and I actually doubt it. It was a theory from other readers. Karas is a vessel that can wield not only the last (not first, i think you confused it with the first dragon emperor) dragon's power, but also other dragons. He is from the bloodline of the first dragon emperor the received the blessjng of the last dragon.
The time travel used the life force (I forgot the exact term used) of the knight, the not-so-boring human, to fulfill a wish from a future generation that needed his help (in this case, Karas's desperation to save Abel in Lil's body). If I remember correctly the explanation from the ice dragon, it means that the knight gave up his chance for reincarnation for that time travel to save Abel.
(Correction)
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Sorry, I think I mixed it up a bit. I think the wish that used the reincarnation priviledge of the knight was the one made at the Spring of Prayers that resulted to Abel reincarnating in Lil's body.
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So abel and karas is the reincarnation of the first dragon and the "not so boring human" REGAS? I didn't read the whole novel but apparently there's a time travel. This is the best thing i've read so far. I must say now it is my favorite!