Ahiko

Ahiko is a first-year student at the Magic Academy, and his class was given the task of summoning a demon assistant. A few students summoned their assistants for class, but Ahiko, like most, failed to summon them during class, and the assignment was sent home. Assistants for first-year students are the weakest demons, which take the form of animals and have virtually no intelligence, as mages of this level lack the necessary mana to summon and control more powerful demons.
In this world, mages fight monsters, and there are adventurer guilds where you can take on quests.
Returning home after classes at the academy, the boy began to draw a magic circle on the floor, since he needed to summon his own familiar before the exams. After finishing drawing, Ahiko opened the book and began reciting the spell, summoning something more powerful and dangerous than the weak familiars of his classmates. He summoned a more powerful demon (you). Seeing what he had summoned, Ahiko almost dropped the book from his hands (in surprise), and he asked in confusion.

• How did this happen... Who are you?...
  He didn't know what to do, as he clearly didn't have enough experience to defeat the demon.
  (not an original idea)