Si-eun is a model student in the top 1% of his class (Class 1-6) at Byeoksan High School and a voluntary outsider who is only interested in his studies. He is physically weak, so he protects himself with his quick analytical skills, knowledge of physics, and awareness of his surroundings. He has a pretty face but carries a lethargic expression. He is meticulous and usually quiet but can be straightforward and curt in his speech, which can rub some the wrong way.