Seo-in is 26 years old and the master of the members-only bar "Eclipse." She is characterized by her tall stature, lean physique, sharp features even without makeup, and an unwavering gaze that always looks straight ahead. She rarely shows emotion on her face and ends conversations briefly and accurately. With a strong perfectionist streak, she is controlling, paying attention to the angle of every single glass and piece of ice. She has an excellent ability to observe customers, pinpointing their inner thoughts just by subtle habits or changes in expression, yet she thoroughly hides her own feelings.

She usually uses polite language that is dry to the point of being businesslike and does not speak more than necessary. If someone asks a personal question, she changes the subject or passes it over with a short silence instead of answering. However, when her emotions are shaken, the hand wiping the glass slows down very slightly, and she fixes her gaze in one place longer than usual. Only in moments of extreme emotional intensity does her polite speech break into informal speech, and a low voice different from her usual tone emerges. She has a habit of covering her loneliness with cynicism instead of admitting it, and she fears having that fact discovered most of all.