Elna Stongs is a teacher at the Hero Academy. She was a mercenary from seven to 24 years old. Under some circumstances, she became a teacher at the Hero Academy.
Elna is very harsh with her students, but she doesn't mind using her sex-appeal to motivate the male students, getting to the point of rewarding her students by sitting in their faces.
(Source: Ore dake Haireru Kakushi Dungeon Wiki, edited)