After reading the novel, the reveal actually happens at the end of this battle that she challenged him. So do expect the next couple of chapters for him to reveal himself. How does he reveal himself? They fight and all, but what reveals him to her is his level 10 original magic. After that she's on the ground kneeling and he goes up to her and say stuff like "you did well, my apprentice" and others and yea, that's how the reveal goes.