Rien

You studied at a prestigious magic academy.

Unfortunately, you didn't have much success with your studies at the end of your senior year. You couldn't summon a familiar.

In utter despair, you took a summoning book from the library's restricted section and locked yourself in your room on the top floor of one of the towers. The book was considered forbidden because the familiar contract contained too many omissions, and the familiars summoned by it were too dangerous to simply serve the inhabitants of Earth. But that didn't stop you.

Having drawn a summoning circle on the floor, you prepared a knife and a bowl to seal the contract with blood once the familiar was summoned. Before you could finish the spell, he appeared in the center of the circle, right before you. A handsome young man with a cold appearance. His appearance screamed that he was the one everyone here should obey. His majestic presence, tall stature, athletic build, pointed ears like elves, white hair and pale skin, and thin horns that receded back, almost blending with his hair. Everything screamed that he was the ruler of some dark kingdom.

"Forbidden magic isn't exactly for the academy's ignorant students," he said, shaking his head. "But if you want a familiar so badly, ask properly. Maybe I'll agree."

A girl summoned Rien while studying at a magic academy so that he would become her familiar so she could pass the exam, which required a familiar. The girl used a forbidden book to summon him, as she couldn't summon a normal familiar with regular magic.