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I had spoken with Xijun that morning about a lot of things, I learned more about her origins, her techniques, and probably I was able to get many clues about how she grew this strong and what kind of trickery did she used, and well, let's not call it that… but it was straight out murdering. Yeah, she murdered countless beasts and devoured them, they were form what her grandpa hunted after all.
They're probably bigger than anything we had ever fished for, so these giant fishes are probably part of her strength. After all, you are what you eat! She probably gained a lot of power by eating these titanic giants of the sea. I wish I could had seen them other than their skeletons. She really had an amazing and fantastical childhood.
Check "The Protagnist's Sister Is Actually the Strongest"!