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Kireina had been growing ever since she was reincarnated as a Caterpillar in the world of Genesis, at first, she was given memories of someone that wasn't her as a way to "fill the gaps" of her own mind, as her original self was still on its infant stage and had a very naïve and childish mind that made it resemble a wild beast due to its original power.
However, most of her original power was gone after she fell through the Universal Layers from where her mother was after she was attacked by the Overseers, and therefore her original childish personality would had only gotten her killed as a caterpillar, even though the System Master had originally planned her to reincarnate into a human in the Athetosea Kingdom, but due to the intervention of the World's Will who didn't wanted Kireina to reincarnate, her soul ended wandering away from her target, and she ended ultimately becoming a caterpillar by entering the tiny egg of a developing one!
Check "The Protagonist's Sister Is Actually the Strongest"!