During the time of the Blue Night, Satan murdered a huge number of the world's greatest holy men, in order to find someone who he could possess without any ill effects to save Yuri Egin, who was going to be executed by the Vatican for being impregnated by him. Shiro Fujimoto and Mephisto Pheles were ordered by Yuri's father, Ernst Frederik Egin, to go find and kill her and Satan's children. They found Yuri within the forest with her newborn twins, whom she named Rin and Yukio , and she soon died of childbirth. Instead of killing the twins, Shiro decided to seal Rin's Demonic powers within the Demon-slaying blade Kurikara with the help of Mephisto, and raise Yuri's children as his own. Mephisto then made a deal with Fujimoto that if he doesn't succeed in raising the twins as normal children and if Rin's Demon powers ever came out, they will be under his custody.
Rin as a child
In his childhood, Rin led a normal human life, but his strength and temper seemed unnatural, so all the children and even teachers feared him and thought of him as a Demon, throwing slangs such as 'Demon child.' He got into fights often and injured all his classmates, and the only one who could stop him was Shiro. Rin always admired Shiro and wanted to become like him when he grew up. Rin always protected Yukio from bullies and encouraged him to chase his dream. Also in his youth, Rin learned how to cook, which became his natural talent.