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Yggdrasil didn't wanted to ask for help, she felt it would be outrageous after all she did to the split souls, the creators of her mothers, the twins. After she treated them so badly, to ask for their help? It felt more than utterly shameless after all. Doing such a thing would also hurt her pride as she still somehow harbored grudges against them. These grudges passed down to her as she shared the memories of her mothers. This also brought their sorrow, and more.
Although over the thousands of years, this changed, she slowly grew into her own self, much like her mothers did as they separated themselves from their main bodies thoughts and became beings of their own, forgetting the pain they held as they had the same memories and emotions as them, including the attachment to people they would never see again.
Check "The Protagnist's Sister Is Actually the Strongest"!