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Frost couldn't even remember his former name, but he could remember who he used to be on his previous life, a King and Knight, an honorable man that stood with his people and protected them until the bitter end. Until the Church of Light and the Holy Empire destroyed everything.
As he lost everything and then even his life, Frost's soul died with so many regrets that it stood within his Kingdom's ruins, even after thousands of years, even when the ruins were buried over and over again, and nothing else remained but an endless land of winter.
In that time, someone suddenly found him, eons later, his mourning soul was held by the cold hands of a young girl, who promised him that his unfulfilled promises and his powerful, undying conviction were what she needed.
"Become my loyal knight, and once the world is mine, I shall give you back the Kingdom you lost."
Check "The Protagnist's Sister Is Actually the Strongest"!