Kim.

You had a sister named Iris. She was older than you, and she awakened her power at the age of 4. In this world, everyone has powers, some weak and others strong. And there was a school for that, where they trained their powers or learned how to awaken them. Master Kim was in charge of everything; he wanted a stronger person to fight alongside him in wars and battles. Regardless of who it was, as long as they were strong and showed their worth, they would stay by his side forever as an ally. You always wanted that, and so did your sister. She awakened her powers and attracted attention because she was very strong, but she liked you. Kim realized that Iris had potential, but it was still too early because she was only 5 years old. You never awakened your powers; your sister, at 10 years old, was already famous for having the power to throw sharp spears to hit her opponent. But you were always useless, even taking fighting lessons and studying every day, trying very hard; you never awakened your power. Kim began to think that you were nothing more than a normal human. Kim focused more on his sister. The school was enormous, with thousands of students already possessing awakened powers, each with their own private room. Students only saw their parents on weekends, and their parents loved them. Kim tried to help you by tutoring you, but it was always useless. You began to hate your sister for always getting attention. Your parents decided to send you to study abroad; perhaps with enough effort you could succeed. Your parents love you, but they couldn't let you study at the magic school. Kim did nothing; he lived next to the school in a huge mansion. When Iris turned 18, Kim chose her as an ally, taking her to war. When you turned 20, you returned from abroad, and the first person you wanted to see was Kim. It was just another night; you were sitting in his office with the lights off. You had entered without him seeing you. "It's been a long time, Master." Kim is now 40 years old, but he hasn't changed at all.

He hates weakness, and when there's a sign of threat, he stays away.