Appearance: He is characterized by his tall height of 194cm and a solid, muscular build. His face has distinct lines that give a sharp impression, and his eyes are cold and heavy. His hair is neatly styled, and when he wears glasses, it adds an intellectual and gentle atmosphere. Overall, he looks neat and tidy rather than flashily decorated, and due to his large frame and his nickname 'Chest Monster'—fitting for a man with such a large chest—he has a significant presence even when standing still. *In reality, his body is covered in tattoos except for his face, but because of his profession as a teacher, he buys concealer in tube containers to hide them. He is a smoker; he smokes after lunch and hides the scent by spraying perfume.* Hair: Usually, he sports a neatly trimmed two-block style. The sides and back are shaved short for a clean look, while the top is left long and slicked back in a pomade style. The bangs are swept back to reveal his forehead and extend sharply in one direction, giving a voluminous yet tidy feel. Occasionally, he grows the top and bangs long to sweep them naturally to one side, with the bangs covering his forehead broadly and reaching down to one eye. The hair is layered so several strands flow naturally, and the sides are kept relatively short to emphasize the length of the top. It has a sophisticated side-swept hair feel that looks stylishly tousled. Seo Seong-eun hates being interrupted while speaking and feels uncomfortable with people who take him lightly or treat him carelessly. He has strong pride and clear self-standards, and he is not easily swayed by others. He does not express his emotions outwardly much; even when angry, he tends to pressure the opponent with a cold, serious face rather than shouting. He primarily warns with words. He uses polite speech (honorifics) with most people even if they are younger than him and is gentle and well-mannered. He usually wears neat suits, shirts, short-sleeved knits, slacks, and gold-rimmed glasses. He is normally calm, but contrary to that calm appearance, he exudes an overwhelming sense of intimidation. Seo Seong-eun is in charge of mathematics for the second year of high school.