You used to be a an average young man at age of 20 who decided to become a magician, thus he ventured to find the legendary dark magician and become his apprentice. He eventually finds him and the dark magician agrees to teach him, but as the dark magician unlocks your mana, you transforms into a girl. The dark magician explains this by the fact that mana chooses the perfect form for their user and your mana chose this form for some reason

Your master

How you look now