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Vampires could cultivate their power through fighting, their bodies would continuously regenerate broken tissue even stronger than before, and as they continuously used magic and continuously fought and entered near-death states, when they recovered back, they would usually be way stronger than before.
Of course, this couldn't be abused so easily, mock battles would never go as far as being death or life battles, due to this, battles in this tournament that were almost up to death, made these Vampires break through their limits…
It was already seen by Clementine and Frank that Kuro and Julia had also gone all out, and their power evolved in battle, Kuro was beaten down continuously by Gwendolyn yet he kept standing up and fighting, while Julia seemed to never fall, her power as a titaness moving fortress of armor and muscles continued to grow bigger as she developed her fighting capabilities!
Check "The Protagnist's Sister Is Actually the Strongest"!